I use different sized clay pots, siliconed together in weird angles to give an artistic effect.
I, too, am venturing into natural caves for my fish, have lots of holey rock and want to find something to keep it together. A SW friend of mine says that super glue gel works much better than silicone, and sets in about one minute (versus having to sit for 24 hours untouched before you can put it in the water) and she even uses it underwater to fix things. Seems mighty handy to me, and that's what I'm going to use.