In a strict RAW reading of the polymorph spell your new form can't use weapons, but this means it can't throw rocks, either.
It is reasonable to permit some weapon use to Giant Apes
Let's assume you can successfully cast Tenser's Transformation on the Glyph, Polymorph into the Giant Ape, and step on the Glyph (I am late to this party, no one else has brought up objections to these parts of your plan, and I can't think of any).
Can a polymorphed creature use a weapon?
Polymorph says:
The target's gear melds into the new form. The creature can't activate, use, wield, or otherwise benefit from any of its equipment.
Although Allan Mills says that it can be argued that any weapon the Ape picks up becomes its equipment and therefore can't be wielded, I disagree. It is clear from context that this passage is talking about the character's equipment, and that it should be parsed as "[Your character's old] gear melds into the new form. The creature can't activate, use, wield, or otherwise benefit from any of its [prior] equipment [since that has become part of its new form]."
However, Polymorph also says (emphasis mine):
The creature is limited in the actions it can perform by the nature of its new form, and it can't speak, cast spells, or take any other action that requires hands or speech.
The "take any other action that requires hands" is a more serious concern. If your Giant Ape 'required hands' to wield a weapon, the polymorph spell itself would then to prohibit weapons use.
What sorts of actions require hands?
The only direct reference to requiring hands in the PHB comes from the text of polymorph, true polymorph, and the druid's wild shape ability, each of which has some version of saying that the new, beast, form cannot take actions that require hands. However, there is no specific action in the PHB that clearly and directly states "this action requires hands", so which actions require hands are up to the DM. There are certainly a number of class abilities that reference hands and might imply that hands are required to use them (such as Eldritch Knights teleporting their weapon to their hand, Monks Deflecting Missiles with their hand, Warlocks recalling their Pact Weapon to their hand and casting rituals while their Book of Ancient Secrets is in hand (and similarly for those with the Ritual Caster feat) and casters performing somatic gestures with their hands). However, as class abilities, all of these actions would be off the table anyway since you can't use class abilities while polymorphed.
Other things that might imply a requirement for hands are wielding a shield (which is "carried in one hand" (PHB 144)), and making a Grapple (but not a Shove) attack, in which you are "using at least one free hand".
However, on the requirement to have hands in order to make a Grapple, Sage Advice says:
Is the grappling rule in the Player’s Handbook usable by a handless creature? The grappling rule (PH, 195) was written for a grappler with at least one hand, but a DM can easily adapt the rule for a handless creature that has a bite or an appendage, such as a tentacle, that could reasonably seize someone. A wolf, for example, could plausibly try to seize a person with its bite, and the animal wouldn’t be able to use its bite attack as long as it held onto the person.
Thus, even for an attack "written for a grappler with at least one hand", the DM can consider granting the attack mode to creatures with other anatomies that are plausible and reasonable.
Do you need hands to wield a weapon?
Nothing in the PHB directly says that you are required to have hands to wield a weapon, possibly because all PC races have hands. However, some weapons have the Two-Handed property. 'One-Handed' does not appear to be a listed Property, but some passages imply that any weapon that is not Two-Handed must be one-handed, for example, "Drawing the ammunition from a quiver, case, or other container is part of the attack (you need a free hand to load a one-handed weapon)" (ibid).
If we examine all creatures that use weapons, it appears that they all have hands, even if those hands aren't exactly human-like; Dragonborn, for example, have "hands and feet [of] strong, talonlike claws with three fingers and a thumb on each hand" (PHB 32) and they have access to all standard weapons. In the Monster Manual, we find thri-kreen with three fingers and a thumb using their unique weapons of gythka and chatkcha, ettercaps who have (at least according to the artwork) just two clawed fingers and no thumbs yet can still use a garrote, and treants that can throw rocks even though their 'hands', like the rest of their anatomy, are indistinguishable from trees when they are not moving. At least for the thri-kreen and ettercaps, it may be that their variant anatomy allows them to use unique weapons specifically designed for that anatomy, rather than standard weapons from the PHB lists.
It seems reasonable to conclude that weapon use requires at least something that can pass as a hand.
But apes HAVE hands!
Apes have hands (under the standard English definitions of apes and hands). In fact, all primates have hands. And while not all apes have opposable thumbs, gorillas do. [I don't know of any official art for the Ape or Giant Ape in 5e, but previous editions have depicted them as gorilla-like.]
In our world, ape hands are versatile, dexterous, and powerful - they naturally make several kinds of tools, including sponges, ant and termite collecting devices, nut-crackers, and spears. Under the influence of humans, they can use their hands to communicate with sign language and lexigrams, to paint, to knap a blade for cutting cords, to build campfires and roast marshmallows, and to use weapons to bring down drones.
Within the 5e rules, both Apes and Giant Apes already wield weapons - they throw rocks. They are the only beasts in the Monster Manual that throw rocks, just as they are the only beasts in the MM that make Fist attacks. It is clear that their natural anatomy allows them to use some weapons.
So let's return to the polymorph spell (emphasis mine):
The creature is limited in the actions it can perform by the nature of its new form, and it can't speak, cast spells, or take any other action that requires hands or speech.
By the rules of the polymorph spell, one can perform only actions natural to the new form, but cannot take actions that require hands. So what does this mean when the new form has hands and can naturally take actions that require hands? That the polymorphed creature is not permitted to take actions that the natural creature can, if those actions would require hands. A bard polymorphed into a Giant Ape can't throw rocks, even though a natural Giant Ape can!
At least to this DM, that...doesn't feel right.
A druid's wild shape, in contrast, says:
You can’t cast spells, and your ability to speak or take any action that requires hands is limited to the capabilities of your beast form,
which specifies that you can take actions requiring hands so long as the beast form already has the capability to do so. Further,
True Polymorph says (emphasis mine):
The creature is limited in the actions it can perform by the nature of its new form, and it can’t speak, cast spells, or take any other action that requires hands or speech, unless its new form is capable of such actions.
This 'unless' clause here seems much more reasonable. Combined with the language used in Sage Advice about adjudicating handless grapples, I as a DM would be very comfortable allowing your polymorphed bard to use weapons as a Giant Ape, and would recommend adding this 'unless' clause to the polymorph spell as a house rule. In a strict RAW of the polymorph spell, however, your new form can't use weapons - and you can't even throw rocks the way a natural Giant Ape can; you would need true polymorph for that.
A DM opposed to this might consider a change of type
If, for game balance reasons, your DM flat-out doesn't want your bard using weapons while in the form of a Giant Ape, that's their prerogative. Rather than arguing about vagaries of anatomy, what the natural limitations of the new form are, and what constitutes a weapon, though, I would recommend that the simplest change is just to declare that Apes are humanoids, and Giant Apes are either humanoids or giants, rather than beasts. This would remove them from consideration for the result of a polymorph.
The Monster Manual tells us that:
Beasts are nonhumanoid creatures that are a natural part of the fantasy ecology. Some of them have magical powers, but most are unintelligent and lack any society or language. Beasts include all varieties of ordinary animals, dinosaurs, and giant versions of animals.
whereas:
Humanoids are the main peoples of the D&D world, both civilized and savage, including humans and a tremendous variety of other species. They have language and culture, few if any innate magical abilities (though most humanoids can learn spellcasting), and a bipedal form.
If intelligence, language, and culture are the gate conditions for beasts to be humanoids, I note that in our world, apes have passed all three, and under the 5e rules Apes have an Intelligence of 6 and Giant Apes 7, whereas some Intelligence 6 monsters that clearly use weapons are Ettins, Gnolls, and Minotaurs. Troglodytes and Quaggoths are Intelligence 6 humanoids and don't use weapons; both Apes and Giant Apes are easily comparable to them. As Dr. Lois Leakey said when Jane Goodall first discovered chimpanzees making tools to fish for termites, "Now we must redefine tool, redefine man, or accept chimpanzees as human."
Fitting your weapon into a 5' cube
Suppose your DM does permit you to wield weapons as a Giant Ape. Your next concern is having an appropriate weapon small enough to be made using Creation. You are worried that a stone maul might end up being too large. Rather than some back-of-the-envelope estimations of the size of the Ape and the size of the maul, allow me to suggest a different approach: use a material denser than stone. Bludgeoning weapons do damage based on their mass, not their size (try hitting someone with a pool noodle and a sap and note the difference). If you want to create a smaller but still impactful weapon, don't make it stone - make it metal. Creation allows you to make objects of "precious metals". They last only an hour, but so does your polymorph spell, so you won't need it any longer than that. I would suggest a platinum rod: Platinum is about seven times as dense as stone, and nearly as hard as iron. Make your Ape a member of the Platinum Club by giving them proficiency in a literal platinum club.
Or, if you have a very permissive DM who is open to off-book weapons, make your giant ape some knuckle dusters out of a nice, dense metal and apply bonus damage to your already-damaging natural fist attacks while still being able to use multiattack. That might be even better than one swing of your metal club!
