This cord extender plugs into the controller port and into the controller giving you six more feet of cord with no hit to performance. If this still doesn't give you the cord length you want, I suggest looking into a
WaveBird controller since plugging more than two of these cords together will cause the response of the controller to suffer.
The connectors fit perfectly into the GameCube and controller, so there are no worries about it suddenly coming loose during an intense game of Super Smash Brothers: Melee.