DurangoSteve:
Yes, I have thought a lot about this too.
Right now, I have a nice nylon tarp that I have adapter to this use. It has loops of rope on the corners and I just unfold and loop it over my Yakima racks up on roof. The racks usually are toward the back and I often put a canoe/kayak on it while loading to prevent damaging the roof. The loops hook over the ends of the rack.
I have extendable poles that hold up the rear of the tarp. Guy to needs. Presto, nice, cheap, small and compact rear awning. Not easy to set up, just a few minutes, but works when parked.
If I have the time, I would love to source out some parts for an easy to deploy, light and compact awning that I could deploy in seconds and use often. Rains a lot of Seattle way!! May get one of those Truck'n Awn units and modify to fit. They are kind of big, but already set up nicely.
Let me know what you find, would love to hear about and see it!
Dave in Seattle