I got one off the other day using the sheldon method
50mm m16 bolt & nut
m16 washers.

You put the bolt through the cup hole, with a couple washers on it and do up the nut inside the shell. With enough washers in there to space the nut behind the cup. Then just keep tightening the bolt. It grips on the cup more and more as it tightens, and eventually unscrews it.
Genius little tool. No worrying about camming anything out or keeping any wrench on tiny thin flats.

illustrates it quite well.