If you're talking about the framerails, your car should now be plenty rigid without cross bracing. Without gobs of power on tap, you're just adding weight for a minimal - perhaps non-existant benefit.
That said, since you're just experimenting, you could possibly set something up like a rear strut tower bar with the opposing thread direction for adjustment.
Seems to me the rails vary in width, so you'd need lots of adjustability and/or various lengths of bar to work with.
