For years now I have been using duct seal to mount my Garmin Striker 4 transducer inside the hull of my kayak. This method keeps the transducer safe, and all the wires and "other" mounting hardware tucked away. I have had no issues with sonar return quality or other issues since using this technique. There are a few other options out there, but this one set me back about $5 and has been flawless.

https://www.youtube.com/watch?v=zis6k2J84V0&feature=youtu.be