This is a simple enclosure that I designed for an Arduino Duemilanove (USB).

The "top" part slides into the bottom one. The one I printed has enough friction that I didn't need any glue to hold the two parts together, yet they can still be taken apart when needed.

There is a hole on the side opposite from the USB port through which you can put cables. There are also two holes in the top part that I used for a led and a push-button. Those can be easily commented out if not needed.

Print both parts.