I have two PA's My PA 12 uses the Lowrance-Ready system. My PA 12 I mounted the transducer "Gooped" to the interior of the hull. The only difference I saw was the shoot through the hull took about 10 min to show an accurate water temperature.
I have been working on submarine sonar systems for the past 40+ years. the plastic of the hull provides minimal signal reflectivity or attenuation.
Its all very boring but it boils down to the speed of sound in a medium. When you see a fish on your fish-finder you are really seeing the air inside the fish Blatter, etc. A fish is like a human, mostly water. When you watch a movie with submarines being hunted the sonar actually get a returning echo off the air inside the submarine. Speeds of sound: saltwater 4800 FPS (feet per second); Steel 16,000 FPS; Air 1,100 FPS. Not sure what the speed of sound through plastic is, But the navy has been using similar products (plastic/fiberglass) they call them acoustic windows.
One other thing I often hear people Ney saying on the process of using goop for mounting. I used it in my outback and it worked without any problems for six years...
Fishing is not a matter of life or death... It's much more important than that!