Try this:

Clean the area with denatured alcohol. Install the muffler as normal. Now, apply red RTV silicone around the area that is opened from the break. It should hold. And even if you don't do anything, the hole is so small it won't make much more noise than normal, it will just add a bit of extra oil around the engine compartment. If you live in a rural area, fly without the muffler. We used to fly without mufflers all the time 'back in the day'.