I put transducer plates on all of my boats, and have for over 30 years. If you install the plate correctly, no water will ever get to your hull or even to the mounting bolts.
I used to epoxy or 5200 them on but after having several come off over the years, at the worst possible time, I now typically use (2) 1/4" bolts to secure it along with the adhesive.
I have used a variety of materials, but in the end, I've been happiest with (#1) 3/4" -1" cutting board, (#2) Starboard, or (#3) PVC trim board.
Trex works, but it swells a bit over time, and always works itself off eventually, and water WILL get underneath it. Don't use Trex, but certain other decking without sawdust in it may work.
If you can find some 1" commercial cutting board, that is the end of discussion. Set it up 1/8" from the bottom, screw/bolt it in with lots of sealant in between. It will squeeze out when you tighten down, and run a finger around the perimeter, wipe up the excess and let it cure for a few days.
Important is using the proper fasteners when you install the transducer. You want to pre-drill it properly, and use a proper, coarse-threaded screw, preferably with the sharp tip ground flat for maximum holding power and to prevent cracking the plate.
I typically have several fish finders on my boats, so having a plate makes adding/removing/adjusting them a breeze without worry. I highly recommend it!