If you built a frame with 2"x2" 's and then covered it with a good quality tarpolin it would be cheaper than plywood and lighter too. I don't know how well this would work if you fish on a very windy lake, but under normal conditions it should be fine. Of course you still need to build a floor of some kind to hold up the frame. I just built one out of plywood and it was very expensive but I plan to use it in the summer for storing boat stuff (gas can, life jackets etc.)
Good luck with your project!