I had a similar issue where I could not pass the club noise test with the standard muffler and needed a larger aftermarket muffler.

I tried a variety of flexible connections and alternative mufflers and that big rattly old thing just broke them or shook them loose. At best I would get about 10 flights and then I .would land with the bits dangling on safety lines. I spent hours fiddling with that setup/

If you can stay with the stock muffler and a short stiff header.