Legal question about AR brace.

Okay so for a AR pistol you need a brace right, what makes a brace a brace? Could i manufacture a brace? Can you 3D print a brace? Does the strap make it a brace? Could you buy a normal 5 position Magpul/B5 and put a strap on it to use as a brace? because we all use those straps and connect them to our arms as we should legally. Does it have to come from a company claiming to be a brace? Can my Magpul CTR wear a strap on it and identify as an SBA3? All answers appreciated, Thanks!