Stopper?

[fieryworld.livejournal.com]

I hope this is crafty enough, cause I thought you guys would be able to give the best ideas...

Well, I'm making a flute out of PVC pipe (the white plastic plumbing pipe stuff) for a science project. First I made one out of pipe that had a hole diameter of 3/4 inch. The flute has to have a stopper of some sort at one end to direct the air flow toward the other end. I was using a rubber cork from some sort of Australian champaigne as the stopper before...

But that flute took a LOT of air to make it work, since it was so wide, and it required so much air. So I made a new one out of PVC pipe with only 1/2 inch diameter. Now, however, I can't seem to find any corks that are the right size for a stopper...

Does anyone have any ideas for what I can use to stick in one end of the pipe that will make an airtight or almost airtight seal that is 1/2 inch wide? Thanks :)