Someone mention Xbox 360 controller hacks? 
There you go

HarumaN is the man when it comes to padhacks. I can't speak from personal experience but I know there have been many people who have used him and will recommend him. I've only seen pics and from what I can see his work looks

and if I ever needed a padhack he would be the first I would contact.
Also to the OP, you might want to consider another PCB I can't say enough good things about which is the Paewang Revolution which works for PS3, PC and 360! Much each to solder wires to this compared to padhacking an OEM Microsoft 360 controller. This way your cabinet will be more versatile and be able to support multiple platforms and most important run MAME!

HarumaN, you should check these PCB out because I'm sure you could sell these pre-soldered for people who aren't very good at it and this should be a walk in the park for you.

The guy who did this found a nice connector to solder directly to the board otherwise when I helped my friend do his stick we soldered the wires directly onto the leads.

Here is the wiring for the joystick or use a small quick disconnect on the other side of the connector. You can also see on the pic above to the very right are the leads for the buttons like Start, Back, Select, and Turbo which presented my friend with some problems but we managed to get it working.