The basic idea was to break up the shock wave while minimizing gas flow impedeance.  I used 1/8" thick 2"1 /2 Dia stainless steel disc inside of a 2 1/8" dia stainless steel tube.  The disc has slits cut from the outer edge approx 1/2" toward the center in 8 regions as shown in the diagram.  Then each "tab" was twised 45 deg from the plane of the disc.  If you looked at the disc front on it look almost as a solid disc with just a small slot area showing through.  The idea was that the shock wave would basically see a "solid or mostly solid" disc where as the gas could still flow through the slots around the tabs.  I had a threaded rod extending the length of the tube with jam nuts on each side to hold the disc in place.

The reason for the rod and nuts was not being a welder-  I used jam nuts - welding it turns out would have been much better as the nuts eventually became loose.  That was not good, when one of the disc came lose it started to spin and greatly  impeded the gas flow.   Although folks told me it sound cool - like a turbin winding up.

In any case, I decided that not being a welder there was no way I could remedy the defects.  I think if there were some way to "spot weld" the outer part of the tabs to the tube and perhas to a rod in the center the muffler would have extend it useful time.  I flew approx 12 hours 

including one trip to Tracy Crook (first extended flight with muffler) by the time I got there at least one disc had broken loose and was spinning.  Tracy was kind enough to use his welding skills to weld the discs to the rod on one of the mufflers (had two one for each exhaust), but within another 6 hours or so disc in the other muffler started to spin.  Also I found that the shock wave pounding eventual would break off a tab or two since they were not anchored and could flex.

Just thought I would throw the idea out there in case it has any merit.

Best Regards

Ed