Xbox controllers for Gamecube?

My Dad likes the giant Xbox controllers, but he only has a Gamecube. He asked me to try and find an adapter to use Xbox controllers on the Gamecube, but all I can come up with is adapters for PS2 to GC, no Xbox. Anyone know if such an adapter exists?

Xbox -> PS2 to PS2 -> GC?

Really, I'm pretty sure there isn't one.