I just noticed that Harbor Freight carries 1 tarp exactly the size of a mini truck bed - if camo is your thing it is a perfect fit. https://www.harborfreight.com/4-ft-6-inch-x-6-ft-6-inch-camouflage-tarp-46409.html
I had a boat canvas shop make a custom tonneau cover out of a waterproof rip stop vinyl material . I went with blue in lieu of black as I think it goes better with my new paint job. If you make it out of the standard Sunbrella material it really opens up your color choices. Its a simple rectangle , snaps around the perimeter, and it uses two fiberglass bows spanning the width of the box. I might have him add two velcro straps so that I could ride with it partially open .
Found on the web, but I saved them to remind me of a good idea. Wouldn’t be too hard to build if you used 1/4-inch plywood, and West Systems epoxy, and made it like you build one of the boats.