I'm looking to create a way for an NPC of mine to become immortal, without becoming undead or requiring the aid of an ally who could die or betray the NPC. The NPC's "class" is not yet final (as in: I could make the NPC a wizard as well as a warlock or a bard. I'm aware I'm not limited to class restrictions, but wizards are different from clerics lore-wise as well, so I'd like to avoid spell mix-ups there).
The Clone spell, an 8th-level wizard spell, allows the creation of an inanimate clone of a chosen creature. Once it's matured, the original creatures' soul can enter the clone if the original creature dies, therefore allowing the creature to "survive" death.
Since this might be the cloned creature's last chance to avoid dying (sort of), it'll obviously want to protect this body, and not just put it in some random basement.
I'm thinking about using Demiplane to protect the body, with the added benefit of protecting a few items that are important to the NPC (this desire might or might not be related to chaotic neutral party members).
The spell basically creates access to a small, cubic 30x30x30 foot room that forms its own, well, demiplane. The plane can be reached using the Demiplane spell itself, or by using other means of traveling between planes (such as Plane Shift or Gate. See also PHB, p. 302). Leaving the plane after the casting of Demiplane used to get there has ended requires one of the latter means.
Which spells, class features, magic items etc. are available to achieve maximum protection for a demiplane?
Of course, Wish can basically trump pretty much anything else you could do, but I'm not looking for "I wish that anyone but me who enters this plane against my will dies" solutions, since that might block you from ever using Wish again.
Related question: Does Clone spell makes someone effectively immortal?