ABOUT THIS PHOTO:
Lying diagonally in the Subaru was possible when the two seat backs of the back seat were folded down, as seen here. The thickness of the seat backs prevented them from lying flat, which was a minor annoyance. Another annoyance was produced by the corrugations of the aluminum panels under the carpeting. That was mitigated by placing a cardboard panel under the carpeting. During road trips the entire rear section was filled with equipment and supplies, so creating a diagonal space for reclining was a nightmare. The flexible sun shade and the things on the floor are unrelated to sleeping in the car.

ABOUT SYSTEM 1:
My first system for sleeping in a car was simply lying on the floor of my Subaru GL wagon. With the back seat folded down, as in the photo, there was barely enough room to lie down diagonally. Try to imagine the entire back section of the car filled with travel gear, then try to imagine moving stuff around and stacking it to create enough space to lie diagonally. It was a nightmare.