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This project intrigues me and I keep thinking how to create a reverse mold of the existing floor. How about taking a large sheet of cardboard that has enough flex an also enough stiffness to conform to the rear section being replaced. Compress it after cutting to shape and get the contours to match. While in place, epoxy or glass on top of the cardboard to make it keep its shape and after that is set you should be able to remove it in one piece leaving you with a reverse of the floor. It would be worth while to place plastic sheet under it in case any resin seeped thru the cardboard. Now when you pour the foam you have a barrier the proper shape to act as a "mold". Escape holes would be needed for excess expansion and lots of weight or bracing on the top side of the cardboard mold would be required. I agree about the wood bracing creating hard spots and would not use that even though I probably would have without reading this thread. Wood will be fine encapsulated in the foam but not good if it touches the bottom and the deck.