The old GEN 2 top hats can be unbolted and gutted. You could probably pick one of those up pretty cheap on ebay. I know it can be done because this is at one of our local dealers for the Precision guy's office.
Now if we can only get a DIY RTK GPS (and integrated roll compensation) setup to speak the proper j1939 messages so the built-in auto steer will function, that'd be pretty nice. I see some industry standard GPS PGNs on the bus but a lot of proprietary ones too.