I have tried making large light-weight boats and a major concern is stopping it from warping!!. I am thinking of using a hollow cored bi-fold door from Lowes, they come in several widths 12", 15", 18" etc, are very light and cheap (especially second hand). Then foam blocks on top and minimum underneath to give it stability and prevent wind flipping it over. Should be able to make nearly 8 foot long but still light enough to handle easily. Any thoughts???