Brings back memories of my hoodlum days, lol. j/k. These days there is no cage, but a bullet-proof clear silicon glass (plastic) partition instead. And the rear seats are replaced with plastic hard seats that have a groove in the back to allow seating with your hands cuffed behind your back. It's far from comfortable. There are, sometimes, however, cage bars across the rear passenger door windows. And the rifle I've seen mounted upright just like that on the partition, other times I have also seen it mounted horizontally on a gun rack mount also mounted to the partition. I guess they figure it isn't accessible to the prisoners as it is on the cabin side of the partition. The partition does have a thin slot that slides open so the officer can communicate with the prisoner in the back.
If you must know, I did time for killing someone.... with my left pinky toe. So I know this on good authority. :p
p.s. you should add the ram bumper guard thingie on the front and also mount more red and blue lights on it. Maybe also add several more radio antennae. And then you should do a duplicate one but make it a K-9 unit (in which case you would use a cage versus a plastic partition) for a police dog. And tint the rear windows.