I took a different approach for the 1" flue on a Pansy charcoal heater. I had made a st/st ring with an ID slightly larger than the chimney.
Then, using one of the wife's silicone baking trays, cut a gasket which was a tight fit over the chimney. The st/st ring has 8 bolt holes and clamps the gasket securely.
It's waterproof, won't burn or char, there's no conducted heat to the woodwork and, most importantly, cheap (until the wife wants to cook quiche).