There are TONS of routes you could go. You could buy premade like Derek's, or DIY obviously. Personally, the DIY route is more fulfilling, even if it doesn't come out quite as refined.
I wouldn't really suggest ripping apart a controller to make the box. I suggest buying a control board. It makes your life easier for very little extra $$, plus it looks cleaner.
The cheapest route is this USB control interface at $24.99:
This includes the board which supports 22 buttons and 2 analogs, all the wire harnesses (makes your life REALLY simple), and even the USB cable.
Also, since this is your first one, there are some good places to get some decent cheap switches. You can try here http://www.allelectronics.com/make-a...hbutton/1.html
As for the enclosure itself, try here. http://www.alliedelec.com/electronic...astic-box-lid/
A bunch to choose from with decent prices.
Last, plan out what you are going to do before you do it... Get a piece of cardboard and do a mock-up of the size and button placement. Make sure the spacing is comfortable and the layout is how you like before going and drilling holes. When you have the figured out, make a paper template that fits on your enclosure and work from that.