I decided to modify this part and use bolts instead of the rivets. The tricky part was to assembly it all together because there is no way to get inside the muffler to hold the nuts when tightening the bolts.
I used different type of nuts which 'locked' themselves against the exhaust face when tightening the bolt. In order to keep them in place at the very beginning of tightening I used CA (SuperGlue).
After all I can tighten the bolts before every flight and easily check them.
The pictures should explain everything: