I repaired a 4" dia. hole left by the original furnace
. What I did was feather the outside surface to about 6" dia. getting it ready for FG.
I had a piece of 1/8" thick polypropylene that I used as a backer on the inside with a some pressure to hold it there. The epoxy won't stick to it.
Then I applied many layers of glass mat/epoxy to the outside until I was just a bit thicker than original.
Sanding, priming and painting
finished like it was never there.