Franco-
Figured if we're taking measurements, we oughtta post 'em, so you don't get 50 people all measuring the same joystick.
Here are the measurements for a Wico leafswitch joystick dustwasher. These are almost certainly good for a Happ Perfect 360 joystick as well, but since I don't have one of those, I wont' say that's for sure.
d1 - outside diameter: 2 inches / 51 mm
d2 - bore diameter: 7/16 inch / 11 mm update: This measurement was VERY snug. I'd go with at least 15/32 or 11.5 mm next time.
d3 - thickness: 1/16 inch / 1.5 mm
Original Wico dustwashers have a dead flat edge profile, but I like B: Full Radius and chamfer.
You've got PM!