1) 1 Empty integrator Phiales(plastic).
2) 1 EuroCent (the important is that the diameter fits exactly the bottleneck).
3) Insulating tape.
Step 1: Hole in the Cap
Step 2: Coin on the Bottleneck
Attach the piece of tape to the coin.
(for better sealing you can also cover the other face with tape as a gasket)
Attach the other part of the tape to the bottleneck.
Step 3: Cut the Bottom of the Bottle
If you want the valve to be more compact you can cut also much closer to the cap reducing the length.
Step 4: Conclusion
This valve is usefull as an extremely cheep(and poor) component for many different projects like pumps(air and other fluids), breathing masks( it has also a nice Darth-Vader-like noise).