I started out with this design that would be copied on all sides, and thus would free a jack shaped object in the center of the cube. Because of all of the abutting surfaces though, the CNC would be unable to mill it exactly like this.

Then I went to a more complex geometric lattice designed to be easily machinable by an 1/8" endmill, but upon arriving to my appointment, the smallest bit to be located was a 1/4". So I changed my design last minute to a simplified ball in box. 

This design did not work either, as the CNC pulled the cube up

 out of the jig and ate into the surface. The resulting form is a bit interesting but not ideal.