Are you guys so original that you don't want to copy what works?  Tracy had that 2nd engine in the Otter that ate mufflers for dessert apparently because it was a turbo engine without a turbo and did not have the Mazda splitter in the exhaust. He finally found a draggy stock box that held up and would be OK if you were putting it inside the cowling, but sure was draggy outside in the airstream. He then went to a Flowmaster Hushpower II which is relatively quiet low drag and held up to his 2nd engine environment. He continues to use it on engine III. Being a copycat, I hung one on the underside of my 9A and am very pleased with it so far. Almost 4 hours of ground running and none of the neighbors have threatened me or the airplane yet :>)

Bernie