ok, I've been messing around a bit in AutoCAD and have got something of a base design. I haven't figured out how to show a view with the measurements yet (first time using the program) but the thumb pad has a diameter of 12mm and is 3mm deep. Ignore the colours and the ring, they're just things to help me when editing at the moment.
I've been considering using rings to increase the grip on the thumbstick, something of a standard practice on controllers when a non-rubberised surface is used. However the GP2x thumbstick is fairly unique in the fact it needs to be as short as possible to limit the amount of travel, needs a very strong grip and also the shaft and thumb pad must be prevented from hitting the casing. What does everyone else think?