C-Base

[Schnabel]

This isn't really from the ground up, but the AndyMark step file is what we are using in the TechnoKats. You can edit it and use it in Inventor, Solid Works, and any other application that has .stp support.
http://andymark.biz/am-0205.html
Go to that page and scroll down almost all the way and you can see that there are two files to choose from, the long and the wide base. Make sure to right-click save as. Also if you want this to make your own variation, I would just copy one of the long c-channels then cut it down as the entire base is made from 4 parts (without the gearbox or wheels).