As noted above - why is it a "requirement" to brace the brace against your shoulder? Because the shooter is thinking within the limited box of it being a "rifle." Therefore if you build a pistol and install a brace to shoot it against your shoulder, you ARE trying to circumvent paying the $200 tax stamp for an SBR. We just need to face it - they sell far more braces than the market for paraplegic shooters.
No different than all the Navy SEAL knives or such - people adopt all sorts of stuff to embellish how they look, not how they perform.
It's a 2MOA gun shooting at 18MOA targets under 125m and more likely 21 feet. Most are 5.56 with minimal recoil. You CAN fire the weapon nose to the charging handle with NO injury. So, why is everybody trying to hold this thing down on their shoulder pocket and then cramping their head down to get to the sight line?
Because most who build one are treating it like an SBR. Simple as that. The answer - don't. Since the toe of the stock doesn't exist to prop on top of our shoulder to raise the sight line up to your eye with head erect, fix YOUR problem. Either Stamp it - or figure out how to shoot it up there. Like a pistol.
That may mean not squaring up - which is a plate method and which contributes to the problem. It may mean figuring out how to use the sling to minimize the minimal recoil the weapon has to the rear, and it may mean thinking out of the SBR "box." Shoot it like a pistol you can hold up to your eye for a sight line with no worry of the slide coming back at your face. Try holding it across your chest toward the off hand with your elbow against the rib cage and using a hand stop to help mitigate the recoil coming back.
All we are really trying to do is stabilize the back of the gun - which, without a stock means you can't hold it into your shoulder. Work around it and find your solution. It's not with a stock like flat surface that is by definition a violation of regulation. Like handgun shooters discovering the Isoceles or Weaver stance, figure out what your pistol stance should be.
A $35 to $125 gizmo isn't necessarily the answer. Ingenuity solving your stability problem is. BTW, my pistol has a bare buffer tube. Nothing on it at all. It shoots fine.