Looking for ideas on making a flood valve.

In Norbert Bruggen's book Model Submarine Technologies he discussed a ballast system called the Aspirated Pump Ballast System where there's an open vented ballast tank with a flood valve in the bottom. To dive the flood valve is opened letting water in and air out the vent tube. To surface the flood valve is closed and a pump empties the tank. The tip of the vent tube should brwak the surface to let air in or else water will reennter the tank.  In the article he even included a photo of a  combined flood valve/pump switch.

Now I'm looking for fresh ideas on making my own flood valve.