I recently came across a company called Graphite Dynamics who are churning out some interesting AR-15 accessories made from carbon fiber including a pistol buffer tube. Their GD-7 Carbon Fiber Pistol Buffer Tube weighs just 130 grams (4.6 ounces). Gear Dynamics claims it is stronger than steel, lighter than aluminum, and quieter too. With it’s 1.30″ outside diameter, it should be compatible with the SB-15 Brace.
It does not use a standard end plate and castle nut. It threads into the receiver without the use of a castle nut.