Thankyou for the idea
pvc would be easier to clean.
What i'm trying to do is, for example, take a tank and cover the inside back and bottom of it with clay (or other material) with ledges and crevices for my geckos that offers places to hide as well as more surface area to climb on. It would be a permanent fixture in the tank.
I've recently read of people using styromphome, scuplting it and covering it in grout.
It was the covering material that i was unsure of. Currently i have a board with a cave/rock display. I bought one of those HUGE caves from the store, but it broke, so i took the pieces, sanded, washed them then siliocned them to a board situated over a rock. It looks really cool and i had planned on hiding the board in the tank with substrate. Now, however, i'm only using tile as substrate so the board is visible and unprotected. So i need some sort of material thats easy to clean to cover it in, hopefully that looks natural too 
I'm not sure if that makes sense.
Thanks again for your help
Red 