This is the one I purchased with and was recommended to go with my new tank from Classic Industries, Stant 10491:
http://www.classicindustries.com/pro...s/st10491.html
I wanted a locking gas cap because the filler neck is at the perfect height and plainly visible to entice any 10-year-old kid to mess with.
This one appears to be adequately vented. There was another thread a while back where somebody said when the tank is filled up all the way, some gas will dribble out during turns. This happens with mine. I wonder if there is a better locking and vented cap.