If you cannibalize something, it should be an extension cord (the only problem is getting those things open to figure out the wire -> pin layout, but all you need for that is an electrical current tester). Cannibalizing a controller is more expensive on your part, and makes the controller less versatile if you get it right (now you can only use it on a computer, Lollermaster, what if you want to use it on your PSX or NES again, huh, WHAT THEN). Also sometime I planned on doing this CORRECTLY with an NES extension cord, and also making something to allow me to use two light zappers simultaniously (haha, little ducks, how do you like my TWIN DUCKIE-SHOOTING SHOTGUNS HA HA HA HA!).
Also when you actually make the cord it comes between using shitty, computer-crashing software that's outdated and barely works anymore (refering to any program that has ever said that it supports home-made controller converters), and programming your own software (which would probably require designing your own pin layout and would be a major pain in the ass, likely).