Find something you carry. In my case, I brought some green plastic bins we tend to store stuff in. When I was younger and my parents were shopping for a car, it was "All four of us and his cello".

Right there, in the lot, we put the items in the car. If it didn't fit, off the list. One salesman tried to convince us that we could put half the seat down, have the cello stick into that area, and squeeze me and my brother into the other half of the back seat... we looked at each other and said "er, no." And left.