These buttons, or more accurately button adapters or extenders, can easily fall off during transportation or if dropped (so I've discovered). 

Rather than buy a new mouse or mouse adapters I grabbed some calipers and got to work in Blender. 

The gcode is meant for Prusa Mini+ PLA. This includes the negative volume for the screw required for the magnet to hold this button in place.

Otherwise you can include your own negative cylinder for a screw you find lying around.

Personally, I found this screw, it has a 2mm shank, but the head was too wide to fit in the gap on the mouse; I had to grind that down to ~2mm as well. A very very small screw is required, or some bit of metal embedded in the print to allow the magnet in the mouse to hold.