If you used a cut down one of those Fernco / flexseal things (upper picture), you could have a GAP between the pipes! So make it all as short as you can. I shld thik you could cut the rubber down to an inch or so, but more woud be better in case it got kicked.
Heat gun solution would about an inch, but practice first so you don't ruin your existing waste!
If it goes brown its way too hot. Keep turning slowly until both parts are cool again.
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