Saw a skoolie on the road once that had a snow saucer inverted over, and bolted to, the escape hatch. Simple and effective, albeit inelegant. It kept the water out and let air in.
I'm seriously thinking about doing this for my bus as an in-storage option.
"You can finally say you have enough horsepower when you leave two black streaks from corner to corner"
(Mark Donohue, famed TransAm driver)