Personally, I'd just slap some aluminum foil tape on there.

Aluminum duct tape (the one for air ducts not the duck tape) is the way to go. It usually has a paper back you need to remove from the tape because it’s super sticky. If you use regular tape, it will unglue with the heat.

I'll echo what others said about the aluminum tape. Make sure it is a good one, not a cheap one. You might want to take a damp rag to the area first. Any dust will stop it from sticking. Make sure to let it dry too.

Mastic could work as well. Get a tub of it and a chip brush or two... and don't forget gloves. It sticks to everything and is rubberized, so it holds up very well. The only downside is that it makes a mess very easily.

Thank you everyone. I'll wipe the area first, then give foil tape a try. I might use mastic for a better seal if it doesn't cost much more.

I have to confirm that mastic plus aluminum tape is the way to go. The mastic will help fill the gap and protect the tape from drying out and peeling away. Be sure to clean it thoroughly so the tape will stick long term. Be generous with the tape as over time it may begin to fail. Mastic is doubly important if you have condensation issues. And if that is the case, or the duct work is in unconditioned space, you should consider insulating and covering the duct work.