I wouldn't use the polyester fiberglass resin. It's got some pretty nasty, unhealthy, and unpleasant chemicals in it, and it's hard to get the right balance of catalyst to resin, and even then, there will likely be some leaching off into the water for quite a while.
Epoxy resin, on the other hand, is some pretty awesome stuff. Very little health risks, and a bit easier to work with.
I've done paludarium inserts/backgrounds with epoxy resin/sand mixes. You can carve up styrofoam or something and coat it with epoxy/sand, or you can just try and mold the epoxy/sand mix by itself. If you want to avoid the glossy 'wet' look, sprinkle on an excess of dry sand/dirt, and then brush off/vacuum off the excess after curing.
If you have access to a kiln, you could make stuff out of clay and fire it, that would work pretty well also.