I designed mine to use 1/4 plywood with a 1x4 frame with stringers every 12" or so. In between the frame members I put 3/4 rigid foam and then topped it off with another 1/4 plywood. As long as I have the cushions in it's fine ... but if you're just on the plywood top it's a little too weak in between the stringers. The ply creaks and cracks when you drop a knee on it.
Whenever you're camping though the cushions are in so I don't think I'm going to add the extra weight of another sheet of ply.
I then bought 2 cans of rubberized undercoating at the auto parts store and sprayed the heck out of it.
Wild Mike