[Buzz]

My advice would be to not bother at all. Roll bars are designed for racing while wearing a crash helmet.
The roll bars on Cobras sit close behind occupants' heads, and combined with low back seats and low cowls, pose a far greater threat to passenger safety than most realize. Most serious injuries in Cobra crashes are from blunt force trauma to the back of the head. Cobra accidents involve spinning out and impacting fixed structures far, far more frequently than capsizing. It's one thing to accept the risk of putting a roll bar behind your own unprotected noggin, but think twice before exposing your partner or passenger to that danger.
In quite a few documented Cobra accidents, the driver has been killed or severely injured while the passenger fared much better. That's because most Cobras only have a roll bar waiting to bludgeon the skull of the individual in the driver's seat.