Now that the (endless) software issues have been taken care of, I built out the controller.

The button on the front edge is the ESC (in MAME) / exit emulator (others) button — press it and the emulator closes, returning the user to Emulator Station.

Lots of wires, but much simpler than it looks. I added an LED (above the GP-Wiz40 board) so that I can confirm that the USB is connected.