I did something similar with a small price of engineered decking. I got the piece of decking for free from Menard's (sample) and picked up a tube of Marine grade adhesive in a tube (not even sure what it was called). It has been glued to the transom, below the water line, for 2 years now with no problem. As BaitCaster mentioned, you'll have to come up with a unique way to keep the board secured until it dries. Also, assuming your transom has some angle to it, you might want to cut a chamfer to one of the ends of the board so that the water seamlessly runs past it without any turbulence. I mounted my speed sensor to the decking and the transducer is mounted with a Lowrance suction cup mount. No holes for me!