The "promuff" muffler i have is made in Palmerston North and they make them to the size etc of what you want.
I told them i wanted a deep noise, nothing loud and he made it in a day or two.
They pack it with a special fibreglass? to keep the noise/sound of what you want and they guarantee it for 3 years.
Had no problems
There ph if you want to ring is....06 3536969