How will these be smoother than machined aluminum? The variation from humidity will probably be more than the tolerance in machined Al. 6-7" is rather large and won't provide very much resistance. I wouldn't bother with a parabolic shape. If anything just make them a bit wider. Since they are totally custom, you might as well build in free-motion, a large flywheel and make sure that the bike is level by offsetting the rear wheel rollers higher.