I haven't tried one of these, but I plan to get one this summer. It's a Fantastic Fan in a box fan frame.
http://www.amazon.com/Fan-Tastic-Vent-01100WH-Endless-Breeze/dp/B0000AY2Z6/ref=sr_1_1?ie=UTF8&qid=1333121703&sr=8-1
Here is a better price, just happened to see it.
http://www.campingworld.com/shopping/item/fan-tastic-endless-breeze-12v-fan/38132