Simple WIP controller made with an Arduino Micro and a Sega Saturn; it involves removing the IC from the Saturn and connecting each button to an Arduino pin.
Currently, I've tested the Arduino with a breadboard and some pushbuttons. It seems to work, however I've not tested it in any actual game, since I'm yet to solder it to the actual gamepad.
Joytick library from: https://github.com/MHeironimus/ArduinoJoystickLibrary