So that's the basics of putting it together, but I've also found a few other nice uses. The first is that when the dog isn't with you, you can stack luggage from floor to ceiling and not have it topple over the back seat. (It won't fall out when you open the door either if you snake the shoulder strap from your bags through the cage wall when you load them.) The barrier is extremely sturdy and will hold up to almost anything, and let's face it, it's not like you're making the rear visibility any worse than it already is.
The second is that you can mount stuff to the wall, and in my case the first thing to be mounted were lights for the cargo area. They're LED lights by Yakima that are designed to go in their roof-top cargo carriers to help with visibility. I DO NOT recommend that you buy these things just for this purpose, as they are like $35 each if you buy them separately from a carrier on Yakima's website. It just happens that I already have a roof-top cargo carrier that I use on vacations during the summer, but in the winter it just sits in the garage. Those lights were already available so I said what the heck?
Pretty simple to mount, the lights already have four screw holes to mount them into the plastic of the roof box, so you can just use those. I bought two thin pieces of wood from Michael's, spray-painted them black, and covered them with felt to reduce vibration. (I probably could have used zip ties here, but they almost certainly would have vibrated.) The LED section is actually removable from the base, which makes a pretty handy flash light. The only unintended issue is that because the lights point perfectly horizontal, you don't get great illumination directly below them. I'm planning on adding some sort of reflective cover over the lights to fix that problem, but haven't decided on what to do yet.
EDIT: Forgot to mention the third thing: you can mount fans on the cage wall to keep the dog cool in the summer. PetSmart sells small fans that are designed to hang on the outside of pet carriers with wire pet doors for $8. I bought two of them and they hang just about perfect beside the headrests on either side. I probably wouldn't buy them again because they require big batteries (I think 4 "D" batteries), have a pretty wimpy fan speed, and go through those big batteries like you wouldn't believe. I'm still looking for a better option...