Have a local tin knocker make you a flat skirt in stainless steel to cover the gaps. Make it the same size all 4-sides so it looks intentional.

If you can’t find someone to make a metal surround like Millwork suggested, try crown moulding on 3 sides.

Patch the drywall and get a better extension that actually lines up with the hood's chimney duct cover.

eta: Since you have ducting inside and are not recirculating you*should not* need those upper vents exposed. Check the manual to be sure.