Foam Stopper 46-50mm

Does the job

I always use a foam stopper like this one in my 2L flask when doing starters. Makes me feel a little more secure than just using foil, which I originally did. The foam stopper keeps anyone from getting in, but is not so tight that a particularly active starter can't push it out with no problem. When that has happened, the stopper just sort of sits on the top, still pretty much doing its job. Thankfully, no absolute gushers yet...

Larger Foam Stopper

This came in handy when I had to use a 2000 ml beaker to start my yeast for an Imperial Red Ale. Easy to use and easy to clean.

Flask stopper

I use it for my yeast starter flask. When swirling the starter try not to let it touch the stopper because that stain doesn't come out.