I got this product used it and even tried to put duct tape around it to block air coming out of the vents. It ended up in the garbage since it wouldn't even stick to the vent. A peice of cardboard would have been more helpful.
These magnetic vent covers don't have enough magnetism to hang onto a wall vent or a ceiling vent. However, Polsteins was very quick in generating a refund. I would shop Polsteins again.