M_Boats: fish finder sensor

Hi, This is probably in the archives, but I can't seem to get there. I want to put a fish finder sensor in the bow storage compartment. What sticks to the fiberglass? I want to stick a 2" dia x 5" piece of pvc pipe to the bottom inside hull and add water and sensor. I can then remove the sensor for use on another boat, which I could not do if the sensor is stuck to the hull. I tried this using silicone sealant and it worked great, for a couple of hours until the water leaked out. So, what is the best sealant for pvc to fiberglass? And, would using glycerin or antifreeze rather than water damage the fiberglass? Thanks, Bob Becker M15 #208