This may take a couple of attempts for me to get the very hub of my gist across, so please bear with me.
Some rule sets for closed cockpit slotcars specify that "2-D" interiors are not allowed.
My friend who is a retired physics professor and slot car racer tells me that "2-D" objects do not exist in real space (or some mumbo jumbo such as that). Even a piece of paper has depth, no matter how small, so physical 2-D objects do not truly exist.
So assuming that the above is true, how do you specify, in as few words as possible, in rules that flat printed paper or cardboard interiors are not legal?