This rule does not affect “stabilizing braces” that are objectively designed and intended as a “stabilizing brace” for use by individuals with disabilities, and not for shouldering the weapon as a rifle. Such stabilizing braces are designed to conform to the arm and not as a buttstock
I used this guy. He is fantastic.
https://www.veritasmachiningllc.com/
For the HK he put it inside the rail where the MP5 A3 slides over. It's very hidden.
For ARs on inside lip of the mag well. Again very hidden.
I have taken everything to him. His work is top notch