When it's hot , I try to be under trees or in my pole barn, often for weeks at a time, so a "fixed solar panel" powered fan wouldn't suit me, unless the solar panel is separate, and can be placed outside in the sun. (That's what I have.) The usual top vent, which you could get, used, at an RV salvage yard, is like an upturned loaf tin. It can be seen in front of, and slightly at the left end of the AC shroud, at http://www.airstreamphotos.com/photo...0654_8-med.jpg
The vent is sealed with Vulkem, where it is attached to the skin.