When I work on my computer from the back seat, I use a tray to hold my laptop. The leveling feet allow the tray to rest on one side on the folded seat and on the other side on the door armrest. The angles of the leveling feet are specifically designed to provide the right tilt for knee clearance and comfort while working on the computer.

I used TPU to ensure better stability for the tray and also to avoid scratching the armrest.