The Box for Arduino is the perfect enclosure for any Arduino project: the internal mounting plastic pins are designed to fit in all Arduino boards.
The box is particularly well suited for use with the Arduino Uno/Duemilanove and the Arduino Mega, with the Ethernet Shield mounted on top. You'll have to break the two central plastic pins to make room for the Arduino Mega fit properly. The sliding slot of the Ethernet Socket is removable, as is the battery slot. A 9V battery will fit in the back, making it suitable for wireless projects.