I've been thinking along these lines myself, but haven't come up with a perfect solution yet.
One option I'm looking into is to buy a real military tarp (OD cotton) and cut it to the right size. It's not difficult to fold the edge and sew it, and a set of 3/4" brass grommets and the tools is like $8. The only trick is attaching it, which I might do with some spare footman loops and 1" webbing I have laying around.
I've also thought about welding up some 3/8" pencil rod into a square and either tacking a piece of sheet metal to the top or again stretching and sewing a cotton tarp over it. Again, the fastening is the tricky part. I saw a guy who riveted some C channel in at the tops of the bed with the openings facing the center, which meant the top slid out but only with the tailgate down.
There's also the Velcro attached tonneaus that I could try to replicate. But I find that sewing velcro strips to vinyl to be somewhat tricky, even with access to upholstery sewing machines, which I have through school.
These are just ideas I have, nothing concrete yet. That's no real help, I know... but maybe you'll be the one to come up with the perfect solution
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