Back in the day people would add an after muffler at the exhaust outlet of the stock muffler. Basically you are just adding more volume the the muffler for exhaust gasses to expand into. It's often unconsidered but a significant source of noise is the propeller. If the prop tips are breaking the speed of sound you are getting a lot of noise-and a loss of prop efficiency. I'm just guessing but with a pusher, I'm betting you are limited on prop diameter. If you use a multiblade or higher pitch prop you can run fewer RPMs for less noise.