This chassis is designed to be used with this battery pack https://www.thingiverse.com/thing:2346235 . This allows this chassis to have a low center of gravity, making it very stable. The gearbox is mounted vertically which leaves a lot of space for electronics and additions. All that is needed is 2 M3 X 10 bolts and 2 nuts.

NB The voltage output by the 6 AA batteries is too high for the motors to handle without using PWM. I use a max duty cycle of 60%.