I have used a "fish finder" in both a fiberglass canoe and a roto-mold poly kayak. I silicone a plastic cup with the bottom cut out to a flat section of the floor, put the transducer and enough water to cover in it. It works fine. My sonar is an older Bottom Line model with a very narrow beamwidth which is good as a depth finder, but not much else. I have a newer Helix 5 DI GPS on my aluminum boat, but the transducer is mounted on the transom.