i dont think he's around to clarify; it sounds like he's.... well, building an external enclosure addon for his vacuum. I'd double wall it, since it wouldnt take that much wood for such a small structure, and just use double osb on the interior. The big trick would be the sound that comes out of the exhaust piping, whether it exits outside the house, or inside the garage. If the soundproofing is for the benefit of the neighbors, make it exit inside the garage. If its for the carpenter inside, make it exit outdoors. Finally, if its for the benefit of the house attached to the garage, it should exit outside as well. The structure should be insulated, and heating may be an issue if its not good for the thing to be in freezing temps(where does this guy live?); but that's about all i know.