What about a whirlybird roof vent?
https://www.google.com/search?q=whirly+bird&biw=1280&bih=861&tbm=isch&tbo=u&source=univ&sa=X&ei=bSGYVJLpF8iHyASG94CYAw&ved=0CDQQsAQ#tbm=isch&q=whirlybird+vent&revid=1083905542
I would be concerned with this pulling too much warmth out with without some heat...