If you are doing the gluing outside the tank - wouldn't clear, non-toxic silicone be a good choice? Seems like a lot of home made decorations seem to use silicone as the adhesive to hold the rocks/gravel on to whatever substructure...
i use plain old aquarium sealant silicone. I wouldnt recommend the use of household silicone, even if it is 100%, because many household and industrial silicones have anti-mold and anti-mildew agents within that can and will cause fish to die.
remember: a "nontoxic" product is non-toxic to HUMANS, not always fish, unless specifically noted on the packaging.
Aquarium-safe silicone from Wal Mart fish section. Let it sit outside of the aquarium for 24 hours before getting it wet. And make sure the surfaces you're sticking together are completely dry. Works like a charm!