This is a modification to clonardo's design which allows the motors to be mounted underneath the robot's platform, parallel to the ground, rather than sticking straight up.
I also include a robot chassis for a small robot made with an Arduino Nano. There is also a AA battery holder, but since you need to purchase the terminals separately, you may be better off just using a pre-made AA battery holder.
This is a motor mount for cheap Chinese gear motors, such as those here- http://www.robotshop.com/en/dagu-gear-motor-pair-90-degree-shaft.html
The gearmotor housing screws to the mount in 3 places, and it's quite solid.
Careful not to overtighten the screws at the top or you may crack the mount.
Print at 25% fill or higher.