When we had an outdoor sandbox, we used 1/4 inch plywood cut to fit the top of the box.
Hubby put hinges on one side, and brass handles on the top on the other side. The boys could lift the top, flip it over and play as much as they wanted. The cover kept the neighbor hood cats and debris from the trees out of the sand, AND the wind would not blow it up or away.
Good luck with your project.
Susan