When I stored my boat at a lake in upstate NY before I moved to Florida, I used 2X12s approx 20 feet long and put indoor outdoor carpet on them and also screwed 2X4s on the edge so the boat would not slide off the side. Added a few 2X4 spacers to keep the width proper and last put a roller on the end of each 2X12 that was at the waters edge to make it easy to pull up on the carpeted area. It took about 2 hours to construct, but what a relief.