For years I've been trying different schemes to block the cold drafts from my home's Air Conditioning vents. They are essentially holes right into the unheated attic immediately above since this is a 1950s ranch-style house.
Because the vents are aluminum I can not use a standard magnetic cover.
The vent screws just go into drywall, so loosening or removing them every Winter will not work. The drywall ceiling just gets messed up.
This year I made covers from white two-ply corrugated plastic. I used Coroplast from old election signs and commercial road spam advertising signs.
Careful now, Mr. Potato Head - don't trespass, or take down signs before an election. In my area, commercial advertising signs on state roads are illegal and so are fair game.
You can also visit a sign shop and ask for rejected signs. Believe me, they have them and will give them away free of charge.