This is an adapter that converts USB controller input to the Sega Saturn controller port using an Arduino UNO + USB Host Shield and two 74HC153s.
I developed and tested it with a PS3-compatible HORI RAP, but I’ve also confirmed that it works with a Mayflash F300 and custom arcade sticks using GP20
40 (Raspberry Pi Pico) with little or no modification. USB gamepads should also work, although I can’t guarantee the button mapping.
It may be a little more involved to build since it requires not only the UNO and USB Host Shield but also a couple of logic ICs. However, using hardware logic to handle part of the Saturn protocol keeps input latency quite low.
If you have the tools and want to build one, give it a try!