The base is made from a vacuum plastic shell that is (was?) available on Astromech.net. I cut out the opening for the wheels and put an RC car chassis in it.

I had to reinforce the suspension quite a bit to support the weight of the droid shell, which was far higher than the RC car was designed for. I put much stronger springs and machined a few suspension bits.

I did change the front wheels too so they would look more like the mouse droid.
The shell needed reinforcement underneath also, in the form of a metal plate bolted to the chassis.