The trouble with installing transducers inside the hull is the installer and the prep work. If the hull is SOLID fiberglass and not the new sandwich fiberglass that has a honeycomb core, the transducer will shoot thru the hull and return a good echo.
I replaced my thru hull transducer last year. I chipped the old one off of the hull w/a wide wood chisel and then used a 50 grit 5" sanding disk to clean off all the old epoxy and remove some of the fiberglasss from inside the hull--1/8". You can use a cutoff wheel.
I installed a base filled w/mineral oil to the hull and then inserted the transducer into the base; it works perfect w/the new Faria gauge.
If you use epoxy the manner of mixing the epoxy is extremely important. The epoxy mix cannot have any bubbles in it that will interfere w/the transmission of signals. Prep your hull first and clean the area and transducer w/brake cleaner. Use plumbers putty to make a thumb size donut about 1/4" - 3/8" wider in diameter than the transducer. Place the putty donut on the hull floor where you want it. DO NOT MOVE the donut because it has an oil base and the epoxy won't stick where it was.
Mix enough epoxy to make a pond 1/8" - 1/4" deep in the donut--LESS IS BETTER. To mix epoxy w/out air bubbles: use a clean flat wood board (luan board works well) and place the epoxy in the center. DON'T STIR it. Use a 3" drywall tape knife to spread the mix onto the board. While turning the board 1/8th turn pick up some mix w/the knife, turn it over and smash the mixture onto the board and spread it a few inches. Continue doing this procedure until the mix is well blended. When the epoxy is well mixed, scrape the mix to the edge of the board and tilt the board and scrape enough mix into the donut mold. Now take the transducer and press it into the epoxy w/even pressure and w/o twisting it. Place a brick or heavy object on it to hold it in place until the epoxy is set or overnight. Remove the donut and its ready to go.