Would be interested to hear whether you think this sort of design is worth pursuing.
I made the ant hill with vermiculite concrete (water:cement:vermiculite 1.5 : 1 : 2 by volume). After setting for 24 hours I took the ant hill out of the jar and misted it several times a day for a week to help it cure. I then carved out the cave with a screwdriver and made a little nest hole leading to the cave from the outworld half of the jar. Outworld is gravel and sand.
Hydration is just a pipette squirt in the nest hole. Will probably put some substrate in the cave, most likely a coir/sand mix, so the ants can customise it and block the hole if they want to. I'm testing it to make sure it hydrates correctly before putting a queen in. A Tar Heel Ants style water tower would probably work better.
I'll also cover the cave front in red acetate or something.