Seal That Vent Cover
We were annoyed to find that leaves, pine needles and other debris would get inside the vent covers through the space all around the bottom of the vent. I added some truck-cap sealing tape to the bottom of the covers, which nicely fills that space.
The tape I used is tape for truck caps and is sticky on only one side. It's a little over an inch wide. Put the sticky side facing up against the bottom of the vent; thus you can easily remove the vent without anything sticking to the roof.