This is a functional two-button USB trackball.

It was made using:

• a 57.2mm billiard ball
• PMW3360 optical sensor
• the PCB from here: https://github.com/jfedor2/rp2040-pmw3360
• Kailh GM mouse button switches (or any other switch with the same footprint)
• this PCB for mounting the switches: https://github.com/jfedor2/mouse-switch-mount-pcb
• KU-B8-OFK (R053010810) ball transfer units ("d" version)
• 2.5mm zirconium oxide (or silicon nitride) bearing balls ("e" version)
• M2x4, M3x4 screws for mounting the PCBs
• M3x12 flat head screws for assembling the case
• a USB cable

I printed the parts at 0.20mm layer height, they don't need supports.