I had to sacrifice an egg to calibrate the ShpereBot and I found out the rubber stopper is only good for plastic eggs and ping pong balls. I propped up 2 bearings on a pile of stuff I had around and that did the job for now. I am going to have to buy a real coupler.
Also note that I used my hot glue gun on the Nerf darts to get them to stick to the threaded rod. There was too much slippage with the much heavier egg.