I recently re-did my airvent, which is hidden away between the aft end of the centreboard case and a lean-to piece of wood that I use as an instrument panel. It's basically a plastic through-hull fitting with hosetail, allied to a jiggle syphon. The idea is that the jiggle syphon lets air in when the tank is being emptied (so that a vacuum can't form so easily) but closes up in the very unlikely event of being inverted with a full ballast tank. For fast filling and emptying of the tank, you still need to undo one of the inspection hatches to equalise the air pressure but this apparatus at least prevents a vacuum from jamming the inspection hatch so that it can't be undone. I'm thinking of putting another one in at the forward end of the centreboard case, in front of the battery.