Max - I have the 50/200 KHz Dual Frequency transducer made for transom mount. I simply stuck it down in a blob of silicone next to my engine stringer where I know I have solid FRP, and it works just fine shooting through the hull. I might be missing some detail or depth range but I get good readings (including individual fish and schools) in 100+ foot water and I don't fish anymore so I don't care if I'm missing some minor nuance. It gives me the structure info and corroborates/elaborates on the simple numeric reading on my Raymarine display.

I got that unit because it came as part of a promo deal and was basically free, so I wasn't picky about the transducer type. I figured it was just a "nice to have". It is. Link below.