Roll bar must be padded anywhere driver's helmet may contact it while in driving position. Adequate padding must have minimum 1/4-inch (6.35 mm) compression or meet SFI Spec 45.1. http://members.tripod.com/~MY5LTRLXCPE/NHRArules.html
section 4.10. sounds like the cheap stuff should work as long as the compression checks out. Hope I was of help. I am in need of a cage myself and have been researching it quite a bit.