I think you would have to go with a prop like Speedracerntrnxie stated, Lower the RPMS and use a more quiet propeller.

Otherwise, you have to go with a canister muffler which probably won't work for you. The other thing is to use a vibration damper system on the engine to airframe connection to help reduce vibrations going to the airframe. The airframe can make a lot of noise too.

Some guys have routed the exhaust inside the cowl into a auxiliary box inside too. They make a wood or aluminum box and route the exhaust gasses into it and the box serves as a second muffler chamber then. It doesn't need complicated sealing or anything. But if you have the room inside that might work for you too.