If you are going to use actual stone, I'd recommend using epoxy or silicone, I think superglue will be more trouble then it's worth, and probably not as effective.
Styrofoam coated with something would probably be a bit easier, and give you more control over it. I've done foam coated with an epoxy/sand mix that turned out fairly well for some paludariums in the past. You may be able to just use epoxy/sand and mold/sculpt it, but that would be quite a bit more difficult.
I'd avoid the fiberglass resin, usually it's some sort of polyester resin, and it tends to be more troublesome then the epoxy resins, and the catalyst is pretty nasty. Getting the mix right can be tricky - too little catalyst and it takes longer to cure, and will likely leach stuff into the water; too much catalyst and the mix runs the risk of overheating and igniting, or leaching catalyst out.