George, here was my  idea of a muffler using cones.  The center (smaller)
tube is perforated with the cone attached to it.  Room at the widest part of
the cone and wall of outer tube for gas to pass.  A variation was to have
the cone extend to the outer tube wall and have all cones facing the header
end of the muffler only permitting gas (and sound) through the perforated
center tube.

Ed

Ed, If I might jump in here.

 This looks like it should work. The muffling won't be tremendous but it should really take the edge off. Bending the shock wave really takes something stout with the rotary, doesn't it? I think the spiro-flow might work and survive better though. (The one that looks like an auger trapped in a can.)  I always worry about perf tube with the rotary, when it gets super hot it seems like the pulses try to tear it off like a perf paper! You really know what you're up against after the washers though so best of luck.

Bil Jepson