If you can lure aerohead into the conversation, great. I'd point to the NASA van and Mair
Something the size of the spare tire in the middle of the back is the opposite of what you want, a box cavity. Consider the Trailer Tails on the back of big rig trucks. They are flat and collapsible because reasons, but something that is curved to follow Mair on the top and side, and flat on the bottom would outperform a half-height boat tail.
Could you put the spare tire on the front bumper?