material to use as a base to make realistic looking wood
I just picked up some terra cotta clay (sculpey) for this exact reason! I couldn't find any info on this product in an aquarium so I am in no way saying it will be safe after a proper cure, but I believe after I properly cure an test I will find that it is safe for fish. I had to resist curing my test run yesterday because it turned out really nice, but the first is never as good as the iterations to come.
I chose terra cotta clay because I will be using these structures as a base to grow plants/moss on. The little clay showing should look natural and not be to distracting.
If anyone has any experience with terra cotta sculpey please share your results to save me any anyone like me the trouble of making something we can't use with livestock.