what about Polymer Modelling Clay? you can shape it and model it and bake it in the oven? I was thinking of doing that, but don't know how safe and fish friendly it is? any ideas?
Other than that you need to use clay with low sulfur content, no manganese and no barium. Look into it further for yourself before you try it. It also needs to be fired to maturity. That's well over 1000 degrees farhenheit. Not something your likely to achieve in an oven. =)
i did make a cave out of a count but none of my fish used to like it ... crab used it more .....
got this cave from a snake shop was my snakes but went in the fish tank and the fish love it, looks tiny in the pic but normally it has a least 3 clowns in it and common plec...