makeblock mBot test stand

The rear metal crossbeam of the mbot is placed on this support.
This allows the wheels to rotate freely in the air without touching the ground.
Ideal for testing purposes for the sort of people who work rather on their workbench instead of descending to the floor and crawling behind the moving mbot.

I've added a version with thicker walls in case anyone wants to print it with visible infill for a better look.