Sand the hull and transducer with coarse sandpaper. Wipe both clean with a solvent like acetone, alcohol or thinner. Use an epoxy designed to stick to plastic. Most don't. Devon Plastic Welder or Loctite Plastix Bonder are two that I have used that work. Use plenty of epoxy to fully fill the space between the transducer and hull. Weight the transducer down with something like a sandbag for a day.