I think that this is a great start for your initial plans. If you have the time/inclination, one way to get away from the "boxiness" would be to play around with ideas of how to make the seat area and the screen area be connected and all kinda "flow" together better. I'm just barely starting to plan out a driving cab of my own, and one thing that's been on my mind is the audio, I really love a cab that has speaks up behind me/on either side of my head, etc. So, it might be fun to experiment in sketch up with some different ways of adding more side panels, etc that go low along the bottom in the middle but then come back up behind/around the seats some to kinda enclose you a bit more....?
Of course, as soon as you start looking at that, then you must make a decision of whether or not you care to easily break down this cab for moving, and if so, then how you can try to create a few diff "modules" out of this that you can assemble/disassemble with relative ease. Adding some extra paneling up around the seats would definitely affect that.
One of the best ways to get ideas, for me at least, is to go to a local arcade that has similar type games and just look at them all really closely (though people might start to steer their kids away from you, if you stare at games that other people are playing, so go during the mid-morning when it's less crowded if you can

).
anyways, I've gotten lots of great ideas from my local "boondocks" fun-center in this regard.
just don't give up! And don't get started until you're 100% sure on your design, cuz otherwise, you'll cut something, lean it up next to the wall/another panel and have a brainstorm of how you "really" should build it, but by then, you've got so much sawdust and sweat expended that you'll just begin to frustrate yourself.

Good luck, and I'd love to see the other ideas you come up with.