You could also go to your local dive shop and ask to see a dry suit (or maybe even a semi-dry one).
They don´t come cheap but have similar valves to help get the air out of the suit as you descend.
You may find something similar and buy/order just that one part.
thats hard if yu want to build something like it. i would suggest a "Luftdruckprüfer" (denn kriegst du ja billig wenn du willst und die größe sollte auchs timmen etwas modifiziert und fertig)