Ten foot Wii controller extension cable already modified into a RetroSpy cable. The cable splice uses 12″ Dupont wires about 1 foot away from the female end of the cable.
This cable also works for the NES/SNES Classic consoles.
For a fully working RetroSpy setup you will also need a programmed microcontroller board. For instructions on setting up a board see https://retro-spy.com/wiki.