I don't think I have the time to make more of these but I can streamline the process for others.
The split aluminum clamp:
Jegs P/N H3R BMR01P
The aluminum stock I got from Speedy Metals
www.speedymetals.com
They are great to deal with and very inexpensive.
The crossbar and standoffs are .75" solid extrusions that I polished.
cross bar is 9.3125" long and the standoffs are 1.125"
To securely attach the headrest plate I made .175" deep hexagonal recesses to trap the nut and then secured by riveting .04" 6061 sheet over the nuts. The headrest is 12" wide and 6" tall, with the curvature scaled from the seat shape. The headrest is attached offset .75" away from center due to the central position position of the rollbar. I tried a number of approaches in my mockups starting with much smaller initial headrest widths but the best balance of aesthetics and protection was to make the headrest match the center to center dimensions of the rollbar and then offset it.
Upholstery is the biggest barrier to "mass" production as it is critical that the hide match the seat, and every hide is different. I left my passenger seat at the upholstery shop so they could match hide, piping, etc exactly. I used Sid's in Mountain View.
Mark