you can go get some cedar 2 x 6's or 2 x 4's and cut them to span the boat hull at the right height at three places or so and it will support the floor. Your going to lay them flat..the opposite of what you would normally do...but the cedar is real strong and the short span they will not be springy. The cedar will last a long time and it does not react with aluminum like treated lumber. The tricky part is cutting the ends to match the contour of the side of the boat exactly so they support properly and don't put stress on the hull in small spots.Take your time and use a cheap 2 x first to get the hang of it. I bought the last ones I used for 20 bucks for 10 footer I think at Lowes. You can paint or stain them for extra protection to water but they should last indefinitely. If you're really creative you can put a thick piece of rubber between the end of the board and the hull.