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I really like the sleeping rig I ended up with for my ModelS 90D:
Two folding tables to deal with headrest area (required a bit of modification to get exact height - four cuts and two holes per table). https://www.amazon.com/gp/product/B07HW2PMQL
These are super compact when folded... I bungee a pillow between the pair.
These tables are useful when spending time outside the car too. Plus, you can just barely squeeze into the passenger seat with one of these tables as a laptop stand if you are so inclined.
I add a couple 2' square interlocking garage mats (I had dozens lying around) and a piece of scrap foam to level the hump.
I encapsulate the mattress in a waterproof sheath that is eliminates bottom sheet and protects against our dog's claws. Easy to clean. I cut holes for the inflation valves so I don't have to deal with a zipper 99% of the time.