It's more "play as a tanky monk" than "make a tanky monk"
What we found at our table is that it's as much a play style as a build.
I'm currently playing a lizardfolk monk as the sole frontliner of our (relatively squishy) party: bard, mystic, sorcerer. I'm following the Way of the Sun Soul, which on paper isn't necessarily optimised for tanking. We've been playing from level 2 until 6 so far, and we've faced multiple deadly encounters in the form of both big isolated heavy hitters and being outnumbered by crowds of enemies – which we all survived.
Deadly. A deadly encounter could be lethal for one or more player characters. Survival often requires good tactics and quick thinking, and the party risks defeat.
Funny detail: my frontliner monk has an AC of 19 (18 Dex, 18 Wis, Ring of Protection), but he has less HP than the mystic (and less even than a wizard I play in another game, at the same level). Yet still I can tank successfully. Below I will explain how it works for our table.
In my experience: that d8 as Hit Die is the only downside of being a monk tank. Keeping your HP pool high can be a challenge, since you already want to max Dexterity for AC (but also for attack/damage rolls) and Wisdom for AC (the higher passive perception is also pretty useful). So raising Constitution will probably not be a priority over Dexterity and Wisdom. It isn't for me, and so far my character has only fallen unconscious twice – of which once due to deliberate friendly fire.
The limited vitality of a monk – taking hits – feels nicely balanced though, as the monk offers interesting alternatives for the classic "tank & spank" strategy. Just like my martial arts teacher in real life (Wado Ryu Karate) used to say: "It's better to not get hit at all."
My advice is to adapt your tanking playstyle to the versatility a monk offers on the battlefield. So, instead of merely "making a tanky monk" I suggest to "play as a tanky monk".
The monk is great for making many attacks in a short time: from 5th level onwards it's 3 attacks per round (or even 4 attacks every round as long as you have 1 ki point to expend). This is Attack action + Extra Attack + Martial Arts (or Flurry of Blows).
When the party is outnumbered, making many attacks could be helpful by turning the action economy into the party's favor: the side that has the most actions per round usually has an advantageous position in combat. So assisting your allies in taking out as many weaker enemies as fast as you can, is beneficial as these foes can't attack you (or your allies) when they're dead.
Against heavy hitters though with lots of health – we hunt dinosaurs – you might want to save your ki points to use Patient Defense instead of going for those extra attacks:
You can spend 1 ki point to take the Dodge action as a bonus action on your turn.
Dodge as a bonus action is huge(!), since it grants disadvantage on attacks rolls made against you. This is especially great if you are in the face of the opponent and your allies can freely focus fire on the target.
Stunning Strike and Step of the Wind could also offer more survivability, combined with the monk's crazy mobility (and adequate positioning of the party).
If the enemy can't hit you or your allies, you'll be an effective tank.
(...) you can spend 1 ki point to attempt a stunning strike. The target must succeed on a Constitution saving throw or be stunned until the end of your next turn.
Step of the Wind
You can spend 1 ki point to take the Disengage or Dash action as a bonus action on your turn, and your jump distance is doubled for the turn.
Make sure to always have 1 or 2 ki points remaining, just in case things don't go as planned.
Also keep in mind that as a dexterous frontliner you have a nice bonus to initiative, a +4 is hard to ignore. My monk usually goes first when combat starts, and this means I can often get into position before anyone else – including the enemies – which is very(!) helpful as a tank. In comparison to the more conventional frontliners such as paladin, fighter or barbarian chances are they are less frequently going first in battle than a monk. (This is unless they go down a Dexterity path too or take a feat, such as Alert to get a +5 on their initiative rolls. Barbarians of 7th level and higher, however, do get advantage on their initiative rolls, which is similar to having a +5 on the roll). Being high up in the initiative order can be beneficial when you want to try make the enemies focus you instead of your allies.
Remember this is a team-based game. Talk with your allies (preferably in character) about tactical options that enable you to be a more effective frontliner for the group. Are there any buffs that can be granted to raise your defenses? Any spells and features that could handicap the opponents? Anyone rocking Hold Person, Bless, Bane, etc?
If your monk still frequently gets knocked unconscious in battle, it could be worthwhile to pick the feat Tough instead of raising your Constitution:
Your hit point maximum increases by an amount equal to twice your level when you gain this feat. Whenever you gain a level thereafter, your hit point maximum increases by an additional 2 hit points.
This is similar to having +4 Constitution, minus the increase for the Constitution saving throw. If the campaign lasts long enough you'll get proficient in all saving throws anyway, through Diamond Soul at 14th level.
Combat Reflexes isn't a feat in 5e by the way, not by default. This feat from 3.5e isn't available in 5e, unless your DM is allowing some kind of homebrew variant of it.