I WAS going to do an x-brace for my senior project during the winter, but my mentor had a baby, and didnt have enough time to help me out w/ the project.
it was going to be something like the picture(I just now threw that together in PS)
I'll get the measurments down for you after I get home from school.
I cant remember, but my mentor said that an x-brace with bars across the sides would be a lot more effective than just an x-brace, and he built a professionally-competitive C5 'vette, so I'd say he know his shit.
EDIT: I used 1inch round tubing(.095 walls), because that is easier to mount shit to hane oval, and I wanted to pad it, and make a camera mount for it too.