Product review (5 stars): Perfect for Return Vents
These work perfectly on return vents.  I did not try them on air flow vents so I can't say if they would hold up on those vents.  I like how they fit and hold.  The white color looks good.  It may be a coincidence but my electric (Electric heat pump) bills have been lower since using them.  The magnets will make it easy to remove from the lower returns up to to the high returns and vices-a-versa.  I would highly recommend these as return vent covers.