You need to be careful on how you skin it in relation to the edge of your plywood floor. You HAVE TO keep moisture out of the edge of the plywood. If water gets into that plywood you are screwed. Seal the edge with coats and coats of paint or use fiberglass resin and run your exterior skin past the plywood. The wood will wick up the water so to you need to raise the floor up even a 1/4" to allow for a space for the water to drain. If you run your skin well past the plywood and even past the flatbed it will help.