Not sure how big a motor you are talking about, but years ago I bought an outboard motor bracket to mount a kicker next to the main motor on my boat. The mounting board was made of micarta about 1" thick. It worked well. My boat had a varnished transom and the color made it look like varnished mahogany at a glance. For a duck boat, I would suggest it be roughed up with sandpaper and painted.