i actually heard on a radio advertorial a few months back that the caps for the plastic coopers bottles are supposed to be reused. obviously you need to make sure that the tops are santized b4 reuse. Chuck em once too grubby or getting out of shape.
the radio show was on 5AA where the resident comedian(John Blake) chucked away all of the bottle tops from his first batch, and in response Coopers sent him something like 500 tops once they heard the story on the radio. nothin like a free plug
...and the bottles are a good size to start a yeast culture.
I have made many cultures in these reusable bottles.
Can also use softdrink bottles for this purpose, as they are available in many convenient sizes