Product review (5 stars): Surprisingly strong magnet
I purchased these to cover fireplace vents that let a lot of cold air seep in. They are strong magnets and it is easy to cut to size.  Unfortunately, my fireplace has multiple entry-points for cold air, but the vent covers do block cold air, at least the air that comes in through the vents. I wish they made this in one huge size so I could put it over my entire fireplace opening.