I used a metal hose clamp to hold the muffler on my Enya .15 IV and it has held very well. I cut it the strap, and then at the new ends I cut the middle of a couple of the "threads'' so I could put the muffler screws through it. Then tightened via the hose clamp screw, not the muffler screws.

Of course, you could also just try lock washers on the muffler screws, but I suspect the hose clamp is more secure. Jim