One of the biggest struggles I have had was regarding what kind of power to use. 

To meet the $10 target,  I could not do anything more than provide a battery holder.

This was good in the sense that it  ran 6V which gave the motors a little extra pep.

The downside was the weight. On the recent versions the the battery pack did not 

fit under the bottom drive shaft. I had recently extended the back.

I made a version which would attempt to doc with a BL-4C battery. The Rev 12  acrylic 

pattern has an attempt at holding this battery in place.  I did not have good luck in 

keeping it in place while walking.  Adding the additional battery boost circuit was also cumbersome. In any case, here is a link to the acrylic. https://cad.onshape.com/documents/b10aa98723fb8ce297ce7b2f/w/d4d43d764bc67b14d7e563de/e/fad4e69b9bd54361983d38ae

I will put the DXF in the files as Rev 12.