Try the coax muffler. Less drag for an external muffler
and a lower note tone. The main problem with rotary noise is
the high frequency component.

It works a little bit like a ram jet, ejector or thrust augmenter
 so it does not add too much to the aerodynamic drag. The tone of the rotary sound is much more pleasant.

Also it can be visually  inspected so it won't plug up if it ever
cracks.Try the coax muffler. Less drag for an external muffler
and a lower note tone. The main problem with rotary noise is
the high frequency component.

It works a little bit like a ram jet, ejector or thrust augmenter
 so it does not add too much to the aerodynamic drag.  Just like a P-51 cooling system. The tone of the rotary sound is much more pleasant.

Also it can be visually  inspected so it won't plug up if it ever
cracks. The outer casing is open at both ends. The inner tube
is cooled by the slipstream flowing between to the tubes..