I knew somebody would know the trick, I forgot now how I got one out many moons ago. It happens to everybody. Those rubber stoppers are notorious for getting through the neck easy as pie. Thats why I now use the "solid" or "drilled" stopper that has a slight lip on the top. I use them on better bottles for the most part but they work with glass carboys in many cases. Usually they seal so well and just the base of the stopper will be tight in the neck it will never "fall" in like a rubber stopper.
These "universal" stoppers mentioned should work on a 6.5 acid carboy from what I remember from mine. Call your LHBS etc and they can tell you for sure.