As for using sandpaper, you would be sitting there forever sanding. If you rough out the cork with a small knife, then sand it will be much faster. Next fine tune it to fit perfectly in the hole. Sound like a lot of work? Probably, but if you want something and can't find it, it might be best to do it on your own-or at least try. It might actually work.
If you want pictures to prove all this, just ask. But I have no idea on how to post them.