I suppose you could try the Hobbico exhaust deflectors, though they have a bad reputation for creating excessive back pressure (power loss), and for falling off in flight, especially when there's lots of vibration.

Why don't you think a Slimline Pitts muffler would hold up to 30% nitro? Last I checked, they were made out of stuff that was fuelproof... Nitro content should have absolutely no affect on the performance of the muffler.