I am sold out of my first batch of GC to Wii adapters that I built. It will be 1-2 months before I have a second, larger batch made.
I got the chance to play around with tiny sf2 sound pad and see what I could do with it. Well I managed to fit a Cerberus pcb inside it to turn it into a USB controller. Quite suitable for playing NES games on the computer.
I've added a gamecube to wiimote adapter to the products page. Since the Wii U doesn't support gamecube anymore, you can use this adapter to use your gamecube pad as if it was a wii classic controller.