I have read not to use an air lock when making a starter as they do not allow the exchange of co2 for O2.
What about these? http://whatman.com/BugStopper.aspx
I can get these as a sample so it would not cost me anything.
They seems like a good idea. I am currently using foil.
My setup is a 2 liter Erlenmeyer flask with stir plate.