Zinc Plated Steel Porous Metal Muffler, 1/4 NPT, 1.20 Inch Long

That's what this one's built for. The Allied Witan P28 uses a cellular sintered metal structure brazed to a metal ferrule, and that porous construction is what does the work of knocking down air valve noise. It handles both exhaust and intake filtering at the same time, so you're not just cutting noise, you're keeping debris out of the line too. Threads in at 1/4 NPT, measures 1.20 inches long, and the 0.56 hex size makes it easy to wrench into place without a lot of fuss.