I re-post this occasionally for information as people sometimes don't want to read through pages and pages of posts:
The space needed for most pinball games is about 31 inches. My 2012 Grand Cherokee has 30.75" clearance and a Stern Spiderman just fits...probably an 1/8" clearance if I remember correctly.
The issue with the 31" measurement is the lip at the bottom of the opening. In my Jeep, the lip is about 1/4" above the floor, but in some vehicles it can be an inch high.
What makes this measurement tricky is that the lip is usually not directly below the opening edge at the top. So measuring at an angle (lip to top) gives a deceiving number, that is this measurement is larger than the actual opening.
If it's important to be able to put a pinball in the back and the opening is about 31", some consideration needs to be taken in the height of the lip above the floor and where the lip is in relation to the opening edge at the top.
Remember that the pinball game needs to ride on the lip while it slides into the back, and it only drops down to floor level after it passes the lip. In my experience, it's the last 2 or 3 inches of sliding the game in when the game is still on the lip and the head is starting to approach the opening edge at the top.