I'm sure I'm to late. But here is my 2 cents worth. I took a piece of Oak flooring (new) 3" long dipped in varnish let it dry then mounted it to transom using 3M 5200. I used tape to keep in place while 5200 cured. Then I attached my transducer. It works fine and I didn't make any holes in boat below the waterline.
Ditto. I've done something very similar on several of my tin/glass boats. I've used everything from sealed AAC pine to a chunk of poly cutting board material. The trick is to use plenty of 5200 and let it set up well. I've never had 5200 fail on me in this application (One install is going on 10 years old.). Advantages of this method is that :
1) you are not drilling holes in the hull.
2) if you make the board tall ( say, 4" wx 12" tall) it not only gives you a place to mount the transducer but also gives you space to secure the transducer wire as it nears the top of the transom
3) can always be removed if the transducer is in the wrong place.
4) allows for easy up/Down/cant adjustment of the xducer to get the best performance.