My finished product (enclosure)

[wagonman76]

The only drawback to the USB is then you'll have 2 USB ports, and if you accidentally plug into the wrong one you might fry something. Other than that, I like it.

[2003protege]

but he used two different sized USB connectors (I can't remember the proper name for those USB shapes).

[Nevyn]

He's got a USB "B" Female jack for connection to the PC (Via standard USB A-B Male-Male cable like printers use) and a USB "A" Female jack to go the the car for I/O and 12V.

USB A-A Male-Male cables exists, but they're pretty rare - that alone will help alleviate any "mis-plugging."