Amiga 500 USB keyboard controller I built for that was a lot simpler in hardware terms, but a bit more effort to implement the serial protocol in the microcontroller.
my Tindie Store.
I also have various single and dual 9 way D to USB joystick adapters that would fit nicely in the A600 or A1200 cases. These allow you to use original 9 way D joysticks (like Zip Stik or Competition Pro etc.) as USB joystick devices, contact me for more info.