Operators usually just cut a hole in the back door, and then bolt some sort of "bubble" over the hole, so that the monitor frame protrudes into the bubble. I've seen them made out of wood, sheet metal, and plastic. Tron/Gorf cabs came from the factory this way, there was a plastic bubble on the back.
The problem with doing it this way is that it makes the game hard to move, because the bubble interferes with the dolly.