Just a quicky review of the removable window vents offered from EuroCampers.com. Maybe other places have them, but I don't know.

http://secure.mycart.net/catalogs/catalog.asp?prodid=4757888&showprevnext=1   

I got into hiking at Mt. Rainier last year and the thought of leaving my van at a trailhead to bake in the sun all day wasn't making me happy. I could crack the window a bit, but flys somehow want in and there is the slight security risk to worry about. Anyway, I bought these vents. Here is a run down of the basics:

The metal is about 2mm thick. 
The slot where the window rolls up into is about 3cm (1 3/8") deep.
The paint and construction is really good and not sloppy. There is a metal screen added to the backside.
The panels are about 15cm (6") tall with a 7cm (2 1/2") x 62cm (2 feet) of actually vent area.  

The vents are pretty sturdy and won't bend easily, but put them in a safe place to avoid sitting/stepping on them. They pop in and out without any trouble. They also give the van sort of an armored car look that tells thieves to move on to the Subaru Outbacks and Foresters in the parking lot.  The picture on their website is a little dark and hard to see any detail, so I took some pictures of my van with a vent installed. They are in the Vanagon spare list photo section:

http://groups.yahoo.com/group/vanagon/photos/album/349533681/pic/75748528/view?picmode=&mode=tn&order=ordinal&start=1&count=20&dir=asc

Yes, I know the moss needs to come off, but this is the Pacific NW! ----Milt ('90GL)