Found a real muffler shop in the next town over and scored a 15" glass-pack for $22.  Jeff, it is only half of you're design, but it was just so easy to do.  My previous setup had a 1.5" length about 10" long as the exhaust pipe, and I just had to slip the glass-pack on and tack weld it.  I doubt that I will fly it like that, but it cut the noise 6dB  at 35ft.

I don't have an Ipod, but my son has a Droid.  Took him just a minute to install a sound meter.  He stood at the mailbox, about 35ft from the prop straight off the nose, and took a reading before and after I put the muffler on at 3000rpm.  It started at 90dB and dropped to 84dB.  Before the muffler, the noise drove the meter off the 150dB scale with him standing next to the cockpit.  With the muffler, it was down to a more manageable 110.  In any case, wit the muffler, we could actually talk to each other with the plane running.

The 3" diameter glass-pack weighs in at 5lbs.

The tune I worked out after the weekend runs also cleared the problem of me smoking the neighborhood with acrid, half-burnt exhaust fumes, because the mixture was so rich that the EGT for the rear rotor wouldn't even get high enough to register.  I now have some places where the mixture goes *to* lean, and some places still where it goes way to rich, but I've at least got a region where it runs smooth and smoke free from which I can expand out.  It starts with only a few turns if I'm in one of those good areas with the throttle, and I'm idling smoothly and reliably down to only 1000rpm.

I STILL have a single oil leak in the gearbox, but it is very small.  More of a seep than a leak.  I can just live with it until I feel like making a gasket, and I won't have to remove to the whole gearbox to install it either.

After the frustration of the past weekend, it feels good for things to just start working right for a change.  I appreciate the offer of the use of your sound meter, Lynn;  but, given the proliferation of smartphones and free apps, it's probably unnecessary.  It would be interesting to compare the smartphones to the meter to see if they agree, though.