The Shell – Switch is a simple block that fits in any of the two Shell MountingPoints (Front and Back of the rover).
NOTE: You can find the main page of the project, with the index of related links and articles here.
The hole has 7mm, and was designed for the switch you can find here.
The main objective for it is to control the power source of your rover, but of course, if you think on any other use for it, go ahead, and let me now if you can, so I can document it here too.
You can see the block, with the switch installed at right of the image.