I just made some last night with gladware bowls, silicone and sand from my tank. They are little huts, the smallest that someone might put dressing in. The larger one is going to be someone's huge cave condo. I also took 2 outlet covers and stuck them together with silicone them so they are like a teepee. I will let you know what the fish go in.
I let mine dry for 24 hours and rinsed. Used aquarium sealant, hope that was long enough.
Black PVC, are we sure it is ok for tanks?