I actually have a setup just like this. I haven't posted pictures in a while, but I should. They have dug it out quite a bit now. I used rotted wood, and like I do with all my formicariums, seeded it with lots of springtails, mites, booklice, and I even have a colony of pill bugs living in there too. mine stays pretty damp towards the bottom and has absolutely no mold at all. When I first made it and it did not have those other organisms in it, the thing was completely covered in mold immediately. Nothing compares to springtails, booklice, and others when it comes to controlling mold. The ants don't ever seem to have any problem with them being there at all either. BTW, the Camponotus I have in that setup are my most successful colony of that species too.