RE: Excelleron 90
This is the one I have. Don't know if it's optimal, but it is very quiet. Significant amount of the noise in this setup is intake and prop. Exhaust is FAR below....
[link=http://www.krumscheid-metallwaren.de/html/gb-extrem-alumininum-schalld.html]KS 76-4[/link]
I think Desert Aircraft and/or Cactus Aviation carries these in the US.