New answers tagged draft
If the worst part of the draft is coming from the vents above and below your insert, you can block the draft with some magnetic vent covers: Test the your insert first with a magnet to make sure these covers will stick.
You may have solved this already but an alternative is a Chimney Balloon. It is easy to install, and you don't see it. If you accidentally leave it in then it will melt and deflate and is not going to go ablaze in the fireplace. You'll need to pick the correct size though,
I have used bondo successfully for several such repairs, with a foam backing. You can get minwax wood filler but bondo will work just as well. You can paint to match after applying.
I think your third idea will work and look the best. First fill the void with expanding foam, allow the foam to cure, then cut it off flush with the face of the existing door trim. Then cover the area with whatever molding/trim you find most aesthetically pleasing. You may wish to miter or cope the corners to get a more finished appearance.
That should work, provided the wife approves of the appearance. The face of the fireplace shouldn't get hot enough to burn your hand, otherwise it'd be a safety hazard. The tape should do just fine. Is the chimney damper closed?
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