I just ripped a couple of Dub City cars apart for their wheels, and found that the orange Eclipse interior features a roll bar. If you pick up this one car, you could get some killer wheels AND a pre-made roll bar for modifying onto the top of your Zip's PCB cover.
Who knows, you might be able to cut up some of the other interior detais to work too?

They cost me 2.69 ea at Target.