I was making a USB adapter for Wii DDR pads, and didn't want to wait for GameCube controller sockets (console-side) to come from China so I made my own. The idea is that you superglue some solid-core copper wires in the channels, and then sand the superglue and wires level with the connector. It's a mess and some of the plastic parts may need more sanding to fit, but it does the job.

If you don't need rumble support, you just need wires at positions 1, 2 and 4, though putting a ground wire to the channel in the flat of the D-shaped thingy may be a good idea.

Start by gluing wires to the center pillar and a ground wire to the flat inner D-shaped ring. Hint: Use needlenose plyers wrapped with masking tape and a rubber band around the handle as a clamp.

Sand flat and clean, beveling the ends of the wires. I bent the tip of the ground wire around the edge so it would get caught on the connector less. Doing this to some of the others might be a good idea, but I didn't do it.

Glue inner part inside the D-shaped thingy. Then there is an outer jacket to provide more guidance for the Gamecube controller plug. The sockets are designed to sit in a cutout in a project box, so there is spacing between the outer jacket and the inner D-shaped thingy.

You need a moderately precise printer for these.