This is a real problem-- and you have to make a decision as to using a restricted in cowl setup or do some wood work to fit up a can shaped muffler.
The quietest in cowl I have seen is the one which comes with the ROTO 35 -but it is restrictive.
We have run it on other brand engines in this size and it is just as quiet -and just as restrictive.
Best?
a short header and JMB 60/60 in fuselage muffler.
This is also more work and weighs more.
I still prefer it.