Simpson Hybrid S 3-Point FIA Head and Neck Restraint 1 "a"
I have just used it in one event with a student that had 3 pt. oem belts. It looks like it does a good job of limiting head movement. I have not used it with a 6 pt. harness yet. Thanks to Marshall of Discovery for fitting it for me.
If you wear it to protect yourself while racing, the DiscoveryParts.com Driver Gear collection is where you'll find it!