Product review (1 stars): I should NOT have ignored the reviews!
I read the negative reviews on this product, and similar products.  I didn't intend to use these while the air is flowing, so I figured they would still work.  They WOULD NOT even stick to my vents AT ALL.  I pressed the entire cover onto the vent, and even though the cover fit perfectly with no cutting, it was as if I was hoping a piece of heavy paper would just stick on its own.  Nothing.

This product is useless.  Don't buy it!