There is also something else that isn't being taken into consideration aside from mechanical fit. There are
load ratings on these devices that will dictate the resistance. Much of the time, the mechanical properties (shaft size, plate size, etc) dictate the formulation of the rubber. In other words, a vibration mount that is designed for a .375" shaft, would probably be overkill to support a 15lb load, but that combination may be what is required to give you the proper feel for the joystick.
It's entirely probable that WICO had a custom batch of these made with exactly the specs they wanted. Even the rubber "grommets" for the Happ 49-ways are made special, with a .313" shaft diameter.
Every stock unit I have ever seen is either .257" or .391", pretty much meaning that a drill will be your best option. Unless you want to to have a thousand dollars worth of vibration mounts on your shelf for who knows how long

.
BTW, this is also most likely the reason that the "knock-offs" reportedly have such a sloppy fit. If the shafts are modeled after the original WICO shafts (mine measures .369") and they are using the commonly available .391" mounts, then there is .022" slop, which doesn't sound like much, but in an application like this, it's tantamount to a "rat tail in a rain barrel".
RandyT