Sealing and insulating drywall within a range hood
While I'd ordinarily reach for polyurethane spray foam, that may not be the best idea on a range exhaust duct that could conceivably have a grease fire inside it at some point in time.

Stuffing the hole/gap with rockwool would be relatively fast and easy, and fire-resistant (more than fiberglass, anyway.)

Slapping a scrap of drywall or cementboard over the open top area might further help, but you might need to dress up the edge of a sheet put there, as it would be visible from the floor.

But it slowly occurs to me that they do actually make a "fireblock" spray foam, and that would be just the ticket here, I think.