Here's how I mounted mine.
1. I cut a 6" PVC pipe about 4 inches long, then tapered one end to fit the curvature of the hull in the desired location. Cut a groove down the top end so you can feed out the cable and put an endcap over the top.2. I then put some cotton under the edges to make a seal and poured in enough epoxy to wet the cotton all the way around and put a book on top. (If you just pour in epoxy, it will run out the cracks, but if you set-up the cotton, it will seal and you can pour the rest in later.) Let it cure for 20-30 minutes. If you wetted the cotton throughly, it should make a seal now. You might be able to use some kind of tape around the inside to make the seal instead of using the cotton to make a seal.3. Next, smear some vaseline over the bottom and sides of the transducer (epoxy won't stick to vaseline).4. Fill the tube to the level where you want the bottom of the transducer to be and check for leaks, it is easier if you let this set up before proceeding because it provides a "level" base for the transducer.5. Add enough epoxy to press the transducer about 1/2 way to 2/3 way into the epoxy before it sets up. Set some type of weight on the transducer to make sure it stays level until the epoxy cures.6. Feed the cable out the groove and put the cap on to protect it.The epoxy will make a snug fit around the transducer and hold it in place. The vaseline also acts as a lubricant so you can pull it out later. If you do, be sure to fill the recess with cooking oil before resetting so you won't have any air bubbles.I installed a cheap Hummingbird 6 years ago. I'm very happy with the unit, but if I leave the boat for a couple of months, I have to pull the plug off the back of the display a few times to clean the contacts.Good luck.Capt. Jim