Product review (5 stars): Works well if trimmed to fit register
We have used them successfully on ceiling registers in an area with strong airflow. As long as you close the register and make sure the cover is trimmed to fit, they stick very well. They can easily be trimmed with scissors or a utility knife to fit the register closely.

But you cannot simply slap on a whole sheet on top of a register and expect it to work. The illustration Amazon shows is very misleading in this respect. If you are not prepared to trim them, don't buy them.

One problem can be registers that have a lot of paint buildup. And of course they only work on steel registers.

If covering the supply registers doesn't cut airflow enough, try covering the returns as well.