Made some experiments with the circuit, and routed a PCB for use with cheap NES controller clones with DE-9 connectors.

The PCB is single sided and needs a lot of jumpers but nevermind, this is just a prototype. It was routed to fit into a CP-011 box.

During the tests it was noticed that the DATA lines need pullup resistors and they were added to the circuit.

Btw, this is the pinout of the famiclone controllers that I have. I believe most of the famiclone controller follows the same layout.