Plain old plastic tarp. I drive a Toyota Tacoma with bed racks...for kayaks. A tarp, pretty much any size, can be a tent, a shade awning,
whatever. I attach it with big spring clamps and guy it out to two poles. Tailgate becomes my table or a seat. With a 12x12 tarp I cover the entire bed and have a shade awning as well.
I used to drive a full sized window van and attached a tarp to the rain gutter with tiny C-clamps, guyed out to two telescoping poles.

For your situation, you could attach a tarp any number of ways, similar to what I do with my kayak racks. Easy and cheap.
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paul, texas KD5IVP