Anything poly. As long as it is clear it should work since air pressure is doing all the work. Just has to be strong enough (couple mm's thick) to take the pressure of being pushed.
[video=youtube;Win_t0xtcQs]http://www.youtube.com/watch?v=Win_t0xtcQs[/video]
I would shoot this dude a quick comment and see where he found his piston and cylinder.