I found a great solution for sleeping in the back of my Model S.
The full size Coleman Quickbed and Quickpump work much better than the foam futon I first tried. The mostly-inflated airbed makes up for the second row of seats not folding completely flat. You don’t have to move the front seats forward to fit the mattress, so one person can sleep while the other drives. The Quickpump inflates or deflates the mattress in about a minute, and they both store compactly in the well behind the left rear wheel. I also bought a 12V extension cord since the pump cord is only 6 feet long and it needs to reach the 12V outlet in the center console. Both mattress and pump are available from Amazon at:
https://www.amazon.com/gp/product/B0741W2B5F
The full size Coleman Quickbed and Quickpump work much better than the foam futon I first tried. The mostly-inflated airbed makes up for the second row of seats not folding completely flat. You don’t have to move the front seats forward to fit the mattress, so one person can sleep while the other drives. The Quickpump inflates or deflates the mattress in about a minute, and they both store compactly in the well behind the left rear wheel. I also bought a 12V extension cord since the pump cord is only 6 feet long and it needs to reach the 12V outlet in the center console. Both mattress and pump are available from Amazon at:
https://www.amazon.com/gp/product/B0741W2B5F