I believe you saw the other thread where I posted these, but a lot of people like to use these squishy tacts for GC style buttons:
Alps# SKEYAB028A. S.P.S.T., normally-open. "Soft-feeling" rubberized pushbutton for pc or breadboard mounting. 7.8 x 7.8 x 5mm. 1mm travel. Orange button on black base. Rated 12 Vdc @ 5 mA, 500m Ohms contact resistance max. 120gm activation force.
www.allelectronics.com
You could either use those and do something along the lines of the 3D printed mounts for the G-Wii rev2.5 (or how Pickles made the custom PCBs for them), or you could make custom PCBs for the conductive membrane from a real Gamecube controller, akin to the Ashida custom PCBs.