Hi, Ed....funny you should mention that....I was just considering ordering a Supertrapp disc muffler which soulds somewhat similar to what you are doing, with the exception that it just attaches to the rear of your pipe.  The one that I am interested in is the 3" diffuser disc unit, as seen on www.http://recerpartswholesale.com/supert2.htm   One of my concerns, however is how well it might hold up, as any parts/pieces of a failed unit would go through my prop (pusher configuration), effectively cutting my flight short and/or ruining my day.  Good luck with flight testing, etc.  Please keep us posted.  Paul Conner