Here's a scratch built robot platform I build when I was a boy. It was build from thin sheets of plastic (plasticard?) glued on their edges. The box structure means it is very light but also quite strong, even so I added a couple of internal struts. A pingpong ball was used as a skid and the wheels were light weight ones from a model aircraft shop. Some off the shelf motors and gearboxes were fitted although I remembers these to be noisy and not particularly efficient.
The idea was that it could be used as a platform for various robot experiments. The control circuitry was never completed and the bot has lived in a box ever since.