Product review (1 stars): THIS STAINS YOUR CARPET!!!
DO NOT BUY ANY TYPE OF MAGNETIC VENT COVER, THEY STAIN YOUR CARPET.

A cheaper and BETTER solution is to remove the vent cover and use aluminum foil or plastic wrap and cover/seal the bottom.  Then, put the vent back in place.  You may get slight air leakage out the sides, but it's still A LOT LESS than these STUPID STAINING COVERS!!!!!!!