A Macs muffler might work but I think it may end up being too long. I think you have the right idea by looking at the various OS mufflers that will bolt up. I also recall seeing a muffler extension (Dubro I think) that came in a 6" length. You cut it to the size you need. Maybe you could use something like that to do a customized extension for an exact fit. You really don't need much.