There is a saying that there are no dumb questions, but with that said, I will most likely feel just that after I get a few answers. Why do you need a wooden frame for your floor to sit on? It would appear that the ply, properly sealed on the bottom and silicone in the bolt holes, could be bolted directly to the frame. This would save you on weight, wood, and ceiling height. Like I read earlier, your mattress is your insulation. You never know until you ask.
Ricky