Try getting concrete debris tarps. Very sturdy, durable and multi functional. It should come in a big roll at a contractor supply house. You can cut what you need to cover the top. Use a grommet set to make holes or weave/sew in a 1/2 braided rope around the edges. The debris tarps are what we use to pull loads of leaves and brush from the back yard to the front(old grunt style). After 4 years we finally wore our first one out.
By the way, Great Set Up!!!!