Kahuna, I (and many I believe) shove a rectangular piece of foam up the vent from the outside, mainly to block bugs out.
I had some black foam which disappears up there. Easy to pull out if you want to (and certainly would take up no room in a bag). I actually prefer the vent open, but got so many bugs and crud coming thru and all over the inside of the shield, and often hit in the face/eyes: I wear DOT halves and good eye wear. Also will keep out big water in a big rain. I have been in a couple big rains, and can't remember if the water comes over the top, probably so at freeway speeds, and with or without the vent being packed with foam. I actually have intended to remove it in hot weather, but have forgotten.
Thanks, that makes sense and is very easy to try. I've got some black foam from an A/C install kit that should work fine. With this way, it will be easy to try it either way without a lot of work. Thanks again!