I have used silicone aquarium sealant for my fish tanks to glue the plants to the bottom of the tank because the fish liked to pull them up, I like it because it's easy to scrape off with a razor blade, however it isn't strong at all and it probably wouldn't work, you might want to look for a sealant made for working on boats or automobiles that stuff is generally strong and water resistant, I don't know if it's toxic or not though... just an idea