I had a similar problem with my OS 70 Surpass, but in my case the muffler vibrated so much that it broke two different header pipes.

The solution isn't really to make the muffler stronger or tighter, but to stop the vibration. To solve my problem I braced the end of the muffler with a copper strip. One end of the brace was attached to the end of the muffler, the other to the plane. The brace isn't to strengthen the pipe, its only job is to stop the unrestrained wiggling of the muffler that creates stress on the header pipe and muffler, and in your case unscrewed the muffler.

I don't know if your cowel would allow this, but if so it should stop the muffler from vibrating loose.

Good luck!