I made some small simple OutWorld humidifiers from mini-terracotta-pots.
I cut down a piece of 'Saugwunder' sponge to a little larger but about the shape of the inside of the tiny pot and then stuffed it in really well until the sponge firmly pushes against the hole at the top hole (bottom of the pot).
The hole makes it very Easy to add water with a syringe or pipet and the weight will keep it down on the bottom of your OutWorld so the ants can't get under it... IF you place it directly on the bottom before adding substrate.
You can make them in any size pot for larger formicariums ofcourse.
The ones I used here are 4cm in height.
Hope you like it and if you make some... add a picture in comments