I don't worry too much about the volume as long as the exits are large enough. But then again as you mentioned noise is not an issue for me. I often convert leaf blower engines, and they tend to have huge mufflers that are 6" or more long and very heavy as well. These are the ones I cut down, gut, and then cap and add exit tubes. I generally use 1/2" or 3/4" copper tubing. It brazes well. In the past I have used thin wall K & S brass tubing, but it is really thin and tough to braze. It really should be silver soldered or silver brazed, but that's expensive!

AV8TOR