Dave,
The lapped split ring is nice, but probably a bit beyond my ability of today...but I could certainly try . I'd have to do the math to get the diameter right, but I can see that would have the nice side effect of making it easy to install as well. I assume when it pinches to fit it will stay concentric enough to seal nicely.
Todd
Its nice if you can turn the ring afterwood to get it truly round, but is so soft and pliable, it seems to seal just fine regardless.....especially with some heat.