This may or may not be the answer you're looking for, but a book I have on building your own homebrew equipment suggested using a larger rubber stopper (the size of the hole) and a series of fender washers and a bolt/wing-nut combination -- SS of course!. They welded the fender washer onto the bolt at the head, inserted it through the stopper's hole, and put a second fender washer/wing nut at the top.
The welded washer/nut combo would grip before it spins, and by tightening the nut, the stopper would expand inside the hole to hold the pressure.
I would think that instead of welding, you might be able to epoxy them together, or at least use some food-grade silicone to make them stick enough to grip before they slipped...
Did that make sense? There are commercial plumbing products you can buy (for stopping up open drain pipes temporarily, etc) that you could look at for ideas. I wouldn't use them next to my beer though...
Hope that helps.
Make beer, not bombs.
(...and not bottle bombs either.)
Primary Fermentor - *empty*
Primary Fermentor -
Secondary - Mystery IPA
Bottle Conditioning -
Keg Conditioning -
Draft: - Apfelwein, Belgian-style Dubbel w/ Grains of Paradise
Bottles:(and still in stock): FIRST AG - (I)IPA 4.1.07, PK's Warm Spring Marzen, Summer Cerveza -BANG, PK's Winter Spice II, Espresso Stout, Indiana Pale Ale, PK's Chocolate Bitter II, Porterfield Porter, and New Year's 2006 remnants.