adjustable seat back support

[RushR.Rentals]

now that the seat is on sliders and needing seat back support with 7.5 inches of seat adjustment at ten different locking positions the common seat back supports with the sliding round bars will not work.
so I started drawing some of my thoughts on how I could make one that will fit in the 1.5" of space from the harness bar to the seat back and quickly extend out to about 7.5" with the seat slid all the way forward.
this is who I have so far.

 

[RushR.Rentals]

the tubing is 1" OD with 1/2" ID.
the side plates are 3/16"

as a control to see the strength I used the SCCA approved seat back brace.
it failed at a lower than expect force load.

when I tested my design my thinking was it would slip on the hinged bolt point/s.
there is plenty of clamping force on the tubing hinges so the spot it started to fail was the 3/16" side bar.
it still took almost 3 times the force than the control test one did.
I could up size the flat bars but at the amount of LBS it took to start to deform the brace you would be dead 2 times over.
the human body couldn't handle the G force loads it would take to deform the seat back support.