Having decided to, one way or another, reconfigure any short-barreled uppers and side-step registering any braced pistols, I was looking to re-use the SB3 brace that I have. I threw it on a 16" M&P 15-22 and I think I will keep it there. I looked at the Split Fix strap as a way to firm up the brace for shouldering, but then was considering carving up a piece of wood to fit inside the void of that brace to make it even more stock-like. Then I thought that someone had probably already come up with a better solution. A quick search turned this up: Devoid Brace Plugs.
For $20, I think I will give it a go. This strikes me as, literally, a guy working out of his garage, but that's fine with me. If anyone knows of a better solution, please advise.