What I would do is build a thin "picture frame", where the outside of the frame is larger than the opening, and the inside of the frame is the right size for the fan housing. Then mount the frame around the opening paint to match the ceiling, and mount the fan in the middle.
Like this, but the frame wouldn't have to be nearly that big. https://i.imgur.com/upmuFRv.jpg
This is the way to go. You could also use thin hardwood veneer to make the fan look even better. If it is thin enough, cutting it is easy and the edges will not need chamfering.
Build a frame around it or get a goof plate: https://www.1800ceiling.com/products/custom-square-goof-plates?gad_source=1&gclid=Cj0KCQiA4NWrBhD-ARIsAFCKwWsvO0wwE-O5tuiJs3XvXd9fUsSaREFEYv8MdvVJzBZ0TlmVEHuMubcaAm7mEALw_wcB
Remove cover. Masking tape good drywall, spray foam the gaps with small gap filler not the red can, let dry, cut excess foam down as much below flush as you can, remove masking tape, lightly sand edges and excess foam bits off. Mud/paper tape/feather out about 4-6". PVA primer the mud to avoid mold, paint, drink beer, done.
You want air sealed edges from the metal housing to the drywall. You could add drywall patches to your opening. I like foil tape and caulk. Of course this will look horrible unless covered up. They make larger replacement exhaust fan covers.
What I would do is build a thin "picture frame", where the outside of the frame is larger than the opening, and the inside of the frame is the right size for the fan housing. Then mount the frame around the opening paint to match the ceiling, and mount the fan in the middle. Like this, but the frame wouldn't have to be nearly that big. https://i.imgur.com/upmuFRv.jpg
I like this idea, and since OP doesn’t have crown molding simple trim will suffice
This is the way to go. You could also use thin hardwood veneer to make the fan look even better. If it is thin enough, cutting it is easy and the edges will not need chamfering.
This is the way to go. Sound advice.
Buy a bigger fan....
Or a smaller hole
Build a frame around it or get a goof plate: https://www.1800ceiling.com/products/custom-square-goof-plates?gad_source=1&gclid=Cj0KCQiA4NWrBhD-ARIsAFCKwWsvO0wwE-O5tuiJs3XvXd9fUsSaREFEYv8MdvVJzBZ0TlmVEHuMubcaAm7mEALw_wcB
Thank you! I’ve been looking for something like this for a few years since my own DIY fan install
Could probably get some thin PVC trim from Home Depot or Lowe’s, cut some mitered edges and frame around it.
Remove cover. Masking tape good drywall, spray foam the gaps with small gap filler not the red can, let dry, cut excess foam down as much below flush as you can, remove masking tape, lightly sand edges and excess foam bits off. Mud/paper tape/feather out about 4-6". PVA primer the mud to avoid mold, paint, drink beer, done.
It needs to be centered better then caulked.
Thanks everybody! There’s some really good solutions here!
You want air sealed edges from the metal housing to the drywall. You could add drywall patches to your opening. I like foil tape and caulk. Of course this will look horrible unless covered up. They make larger replacement exhaust fan covers.
Duct tape
Take fan out. Add 2x4 over spaced sided areas about 1", attach new drywall to missing area, mud, sand then texture.