Meguiar's heavy duty headlight restoration kit, for windshield.

I have some of the pop-out vents that I have been very tempted to try this on...

 

I know where you can get a pair of Snapvents for $15.85 a pair.
http://www.aircraftspruce.com/pages/ap/airinletkits/snapvents4.php
These mount in a 3-3/16 hole so you'll need a 3-3/16 hole saw to install them. If you are good with a jig saw you could cut the holes out that way. The best way to do that is to draw a 3-3/16 circle using a compass on paper and attach that paper circle to the place where you need the hole. Then cut it out being very careful not to go outside the line. Then clean up the hole by using a round sanding drum to sand the hole out to the line on the paper. Then remove the paper, clean up the windshield, and snap in the vents.

 

It depends on how fast you are going. If your tooling around the neighbourhood at 30 or less they don't do much to keep you cool in hot weather. At 40 and above a nice amount of air will flow into that area behind the windshield where you're sitting. They will keep the inside of the windshield from fogging up. If you rotate them 180 degrees so their "Backwards" before you close them rain will not get past them. I never bothered to close them even when it was raining. I would get wet anyway so when the rain quit the air flow through them would help to dry me out. There was also no need to stop to close them and then open them after it stopped raining. I don't bother with rain gear. You're still going to get wet and you'll never dry out until after you stop to take off that rain gear. Then you've got wet rain gear to pack and deal with later.

 

I find that the vents aren't good for letting a lot of air in, but they are great for is equalizing the area of low pressure that is behind the fairing which helps decrease helmet buffeting.