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.