Ok, I happen to have a lathe at my disposal, so it's a little easier for me to make one of these really quickly.
The I.D. is .5195 (just a couple thousandths over the size of the joystick stem)
The O.D. is 1 1/8" (just a couple of thousandths under) to match the hole drilled in the control panel
Put it over the joystick stem, put it in your control panel. Now the joystick is centered in the opening perfectly and can be mounted to the control panel.
Remove the bushing and install your dust plate and knob.