Product review (3 stars): Very thin, but they work
These magnets are so thin, when I first pulled them apart I couldn't tell which hand was holding one or two.

I cut them with regular paper scissors to fit the two vents I wanted to cover, and used a hole punch to make holes for the vent screws (in order to get a good, flush contact) and have not had a problem with them falling off.

I put one on a vertical exhaust vent and the other two on a large vertical HVAC vent.

Would probably be fine on a ceiling vent if supported with extra magnets. Neodymium magnets, hopefully.