Product review (5 stars): I Used Mine To Stop The Cold Air From My Fireplace Blower
In the winter, when there is not a fire going, the outside air chills the fireplace heat box and this causes a back draft through the blower intake. Even with the top vents closed, air is still pulled into the blower area, chilled, and comes into the house through the intake vents at the bottom.

These magnetic covers were the perfect size to stop this.

Yes, they are kind of weak. They definitely wouldn't stay on a vent on the ceiling, and would not stay on these vents if there was active airflow. But for my purpose they were perfect.

Now I just need to hit them with some black or brown paint and they'll be worth 6 stars!