I want to air-seal my fireplace. The damper is 44" wide, and it's about 4 feet off the floor, not easy to reach. The chimney tapers gradually for several feet after that, a couple inches narrower per tier of brick.
I'd like to install a chimney balloon, but they max out at 38" wide, and they need parallel surfaces to press against. My chimney doesn't have anything fitting those constraints until somewhere near the ceiling.
Update: I won't use the fireplace, but some day I'll sell the house, so any blockage will need to be reversible.