If the muffler is fully enclosed in the cowl an extension tube of the same size as the exhaust outlet will probably work. I would attach it with a small sheet metal screw. Your cowl should be fairly easy to remove so that you can check to see that the screw remains tight after each flight.
One of my main complaints about 4 stroke engines is that I have never been able to find an extension that fits between the muffler and the engine for cases such as you mention. I suppose there are such mufflers, altough I've never found one that will allow the exhaust to be completely cowled.