We just modified our Sparco Evo Plus to use a seat back brace. We made the whole thing our selves. Yes composite seat - a Sparco Evo Plus (roughly the Evo 2USA or 3)
As DC Doug points out, the seat back just has to be 6" from solid (roll bar or roll bar like) part of the roll cage. So you could make some sort of adjustable roll bar piece that is in the area of the seat back. But this is probably more complicated than just adding a seat back brace.
There has been some discussion on seat back braces and the small foot print of the brace on the seat back. Some fear of having the brace foot shear through the composite in an accident and break the spine. So the solution is to use a bigger plate on the seat back, spreading the load and reducing the risk of shearing through. (which by the way, is nearly the same risk as fracturing over the roll bar which is <6" away - Think about that).
What we did.
Got a piece of .120" steel, about 10"x6". Took the seat and the plate outside to a dirt area with a sledge hammer and John S. pounded on the plate in the dirt. The dirt acted like a shot bag, allowing the blows to bend the plate, but not very much allowing for better control of shape. John got to within about 1/8" overall shape to the back of the seat, shoulder area, just below the seat belt cutouts.
We had some 1" square, 0.125" thick wall Aluminum tubing which we drilled a hole in one end, and then rounded the end to allow for pivoting. Then we made 4- 1" steel angle pieces 2 as pivots, and 2 as the roll bar pin pieces (pre-drilled for cross pin/bolt). We welded the pivot pieces to the plate. Lined up the plate and AL tubing in the car, and marked the seat. We undid the seat cover and then match drilled plate/seat with 8 small pilot holes. We were using some 8-32 Stainless Steel CSK head screws I had around for another project. The countersunk screw heads would be nearly flush with the inside seat surface and so not pose any risk to the drivers back.
We tapped the plate for the 8-32 screws, drilled and countersunk the seat for the screws and installed the back plate. We added nylock nuts for extra locking on the 8 screws. Put the seat in the car, lined up the Aluminum tube with the roll bar and clamped on the two 1" angle pieces. Those got welded in on the underside of the harness bar, and then we marked and drilled a couple locations of the seat to fit our various drivers.
A note on screw size and composite seats. Composites (fiberglass and carbon fiber) are strong and tough in continuous form, but when you start putting in screws, the local stress/strain can easily exceed the strength of the material. More smaller fasteners are better, the load on each fastener is small so its less likely to rip through. More of a small loaded fastener equals the same strength as one or two bigger fasteners. We used a 3/8 bolt for the cross pins of the brace. So that is 0.375" DIA, or 0.11044 sq inches of steel. A #8 screw is 0.164" DIA, or 0.0211 sq inches. So dividing 0.11044/0.0211 = 5.2 . Meaning 5.2 of the #8 fasteners is equivalent to 1-3/8" bolt for strength. Now I did a real rough job of estimating, and Stainless Steel is not as strong as regular steel, or high strength bolts (grade 8), so put in a couple more for safety and you should be OK.
The other thing to remember is the seat back brace really only takes a backward load. Meaning the driver gets slammed back into the seat, if the car is hit from behind. If the car impacts a wall, and the driver is thrown forward, the seat belts take the load. So the seat back brace just needs to support the nearly direct load from the seat to the roll bar, and the shearing forces of any angled bracing that are going to try and peal the brace off the seat back (because the brace will not be perpendicular to the reclined seat back). And the wide plate and brace tube are really doing most of that work.
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?Everyone who has ever built anywhere a 'new heaven' first found the power thereto in his own hell- Frederick Nietzsche