Yep, Velcro for the win. I use a lot of the industrial Velcro from home improvement stores (for various projects), which is very strong, but which has issues with the glue lasting in hot temperatures. I would sew the Velcro loop side to the canvas top, which may require an industrial strength sewing machine, or to outsource the activity to an upholstery shop. Rivets and glue work well for adhering the hook side material to the metal. My truck tonneau cover, which uses Velcro along the sides, is designed to keep water out by having the edge at an angle (about 30 degrees).