I would try a product called Plumber's Goop. Made for fixing plumbing, so it is waterproof. goes on like silicone, but dries very hard, like an epoxy. I used it to mount a plastic cup in my kayak to be filled with water and mounted my transducer in it. Has been working for over 5 years. Comes in a tube kind of like tooth paste size, and is resealable for future repairs. Strong stuff, follow the instructions.