2 layers21.6 × 52.3 mm

Ps2 pico hat
No0ne/ps2x2pico
USB keyboard/mouse to PS/2 interface converter using a Raspberry Pi Pico
17 parts399MIT
KiCad boards from 2 GitHub repositories tagged “usb-mouse”, the most starred first; open any of them in PCBKit.

No0ne/ps2x2pico
USB keyboard/mouse to PS/2 interface converter using a Raspberry Pi Pico

doj/Ultimate-USB-to-Controlport
Arduino project to convert USB joystick or mouse to digital for use with Commodore or Atari computers