The radical solution is to get a piece of all-thread rod from the hardware store and a handful of nuts with nylon inserts to fit the rod. Disassemble the muffler and drill the holes in the muffler as necessary to run the rod through the muffler. Reassemble the muffler w/ the all-thread rod in place of the stock screw, trim the rod as necessary and add two nuts on each end, tightening securely. You'll never have that problem again ... and it's unlikely you'll ever lose the back half of the muffler either. Total cost about $2.50 and 10 minutes. One all thread rod is enough to fix 2-3 mufflers.

Live long, and prosper
Jerry L. Gubka