This Panel Mount Dual Power Cable is great for adding a external jack to any Arduino enclosure like the Project Box For Arduino.
This cable splits from the single female DC jack (2.1*5.5mm) into a male DC jack (also 2.1*5.5) as well as two .1"/2.54mm pitch female jumper cables. The male DC jack will plug into most Arduino boards, leaving the extra power cables available for motors, relays, and more!