The ultrasonic module consists of 7 HC-SR04 distance sensors and a 3-D printed enclosure. The module will be mounted on the front of the robot base, a second one can be mounted on the backside if necessary. So far I finished the 3-D drawing of the enclosure.

3-D printed part from Shapeways:

7 HC-SR04 mounted:

I also designed an extension shield for the two Arduino MEGA's to have better access to the remaining I/O and analog pins:

Populated extension shields for the two Arduino MEGA's, MARB has on board:

Wired and tested ultrasonic sensor module: