Run to the hardware store and get a 4-6 inch section of a PVC pipe. Also pickup plenty of SLOW!!! curing 2-part epoxy (24hrs is best). Diameter of the pipe should be large enough to fit your transducer. Usually a 4'' diameter does the job for garmin and lawrance. Find a spot on your hull that will least likely have a layer of foam between 2 layers of fiber glass. That spot is usually located right infront on the sides of or behind the engine mounts, since those are the areas of the hull that have to take on the most stress and therefore are more likely to have solid fiber, with no foam in between the layers. Cut one end of the pipe to the same angle as your transom at that spot. Meaning, if your boat is a deep V, and that spot is on a 20 degree angle, that angle has to match the angle of your cut, so when you put the pipe section down, the top of it is perfectly horizontal in relation to the ground. (PM me if more clarification is needed on this step).
Now, position the pipe section on your hull, and make a relativly watertight seal all around it (on the outside) with good acrylic or silicone caulk. Mix a bucket' o poxy, and pour about 1/4 inch into the pipe (already positioned and sealed so epoxy wont leak from the bottom.) Gently place the transducer into the pipe, and epoxy, using a slight twisting motion until transduce head almost touches the hull. Use slight twisting motion to work out any bubbles. Once positioned, pour the rest of the poxy into the tube, very slowly, avoiding bubble formation. (NO BUBBLES= THE KEY TO SUCCESS). Do not layer it, pour it non stop until the head of the transducer is either almost or completely covered. Secure it all with a couple of small sand bags or whatever other material may be handy. Let it cure. Do No Take the boat out for 24 hours or you will have a wasted transducer.
Works like a charm up to 35 knots if properly done. Unless transducer uses low power or of poor quality in general.
P.S. Fit the pipe to whatever size you feel won;t interfere with stuff in your engine compartment. As long as the transducer is fully or almost fully submerged, you are fine