The first proper trailer may have stolen the show at Star Wars Celebration, but the revelation that spherical droid BB-8 was a physical prop came a very close second. Like a lot of people, when industrial designer Christian Poulsen saw the adorable sentient ball roll onto stage, he decided he had to build one of his own. Unlike plenty of others, however, the BYU student was able to design and build a working copy the following day. Turns out that you can too, as long as you've got a Sphero and a CNC machine lying around the place.
After splitting a first-generation Sphero in two, all Poulsen had to do was attach a neodymium magnet onto the robot's stabilization arm. Then he could just craft a dummy head with a similar magnet on the inside, as well as some felt to ensure it could roll along the Sphero's surface easily. The only other thing anyone would have to do is provide the empire-approved paint job and you're good to go.
There is one downside to this, which is that the extra head will unbalance the Sphero's built-in gyroscopes. As such, your ersatz BB-8 will only be able to travel in one direction unless you give it a helpful bump to turn it around.