Product review (2 stars): Magetic strength too weak for steel ceiling vent
I bought the product to cover steel AC ceiling vents to block draft and cool air.  The product appeared too weak to hold itself up.  It may have worked better after trimming product to the grill size, but I did not want to void my ability to return them if they failed to stay up.

I would recommend the Elima-Draft covers as they are insulated, better looking and easier to install.  Unfortunately, they do cost more money.