[Drosophila] preventing fruit fly escape in container?

[Bill Washerman]

I am starting a fruit fly colony for various experiments.  Once the colony 
is established, I need to retain the fruit flies in the container. 
Initially, I'll simply have a standard window screen covering the opening, 
but eventually I want something with finer mesh that will allow them to 
breathe, but not escape.  I hear tulle fabric is a possibility- can anyone 
confirm this or suggest something else that's cheap?

Thanks in advance,
Bill Washerman