I used 1/2 inch plywood. I drilled about 2 inch holes with a holesaw in each chamber of the hull. I drilled them about every 3 feet. I tilted the boat nose up so the foam would run to the back. Measure foam parts, mix and dump, allow the foam to expand and start to set before i made another pour. . I had tabbed the plywood to the hull around the edges, poured all the foam, put wood plugs back in the hole, fiber glassed them in and layed in a thick layer of fiber glass cloth, built it up and was ready to go. It was an awesome little fishing boat. 25 hp would zip you along nicely. Set up with stick steering would be even better.
The love for fishing is one of the best gifts you can pass along