Steve:
You might consider modifying the stock muffler rather than making one up from scratch. The muffler in your pictures looks very different from mine, but maybe the ideas would still work. I ground off the crimped over part that held the muffler together and removed all the stock "guts". For the outlet, I used a thin wall steel tube with a 5/8" I.D. (about 16 mm) which I think was a piece of electrical conduit. I reassembled with silver solder which is a form of brazing. Picked up over 400 RPM from the stock muffler, and it doesn't seem to be too noisy, but I haven't flown it yet.

Check out this site, "Teaching a weedwacker how to fly" if you haven't already seen it.
http://www.geocities.com/CapeCanaver...216/index.html

Here's a couple more pics of the economy starter for Mike. It really does work, and you don't have to get as cozy with the propellor as you do with an electric starter.