Theoretically, converting a SNES control pad to work on Xbox wouldn't be hard, provided you had a SNES controller and an Xbox controller to gut and merge... just wire the controller chip from the xbox pad into the SNES pad; they have plenty of room inside the shell to hold excess wiring and components.
I gutted a cheap pc usb gamepad and wired the remains into one of my SNES controllers, to make a SNES USB gamepad for use with emulation... I can't imagine it would be any harder with an Xbox controller...just go buy a used, cheapo 3rd party pad and gut it.
As for constructing an original device... that's beyond my expertise.