This part is a hollow cube with a ball that is free to move around inside, but cannot come out.

I have always liked these kinds of things. It is such a simple design, but it is so cool. I keep one on my desk and everybody who sees it has to pick it up and play with it.

Once printed, you will need to cut off the support for the ball on the bottom. Also, as you may know, the Cubex Duo is not a very good printer, so I apologize for the roughness in the pictures. Printing on a Makerbot or other decent printer should yield much cleaner results.