You might want to look into a "throttle pipe" from justengines or westonuk.

A throttle pipe will give an increase in performance over a standard muffler, dramatic noise reduction, linear throttleability, (non-peaky) as required in 3D and funfly.
Email: [email protected]

You can buy one "Throttle Pipe 60 / 75 tapped" for the exhaust spacing used on the Tower .75, attach it to your engine and go fly.

WestonUK also make them.

You can also make one from one of the aluminum "beer/softdrink" bottles as I can't find anyone here in the states that make them commercially....

You wouldn't need to cut your header as the "beer bottle" just slides front to back like a trombone slide to tune. Try-it-n-Fly-it...kind of thing.

http://www.westonuk.co.uk/westonuk2_193.htm

I hope this helps.