A Japanese-style spherical ice mold, as suggested by http://www.youtube.com/watch?v=cuwJEN3VG-Y and documented at http://blog.ussjoin.com/2012/01/making-the-mold.html. Made from aluminum, you just heat it under running water, then insert a block of ice: it simultaneously melts and freezes the ice to form a perfect, seamless sphere.

Tool: 3-Axis CNC Materials: 2 3"x3"x1.5" blocks of aluminum

You can make the blocks of aluminum approximately 3"x3" (out of 1.5"-thick billet), then do the milling, then do a final pass with a manual mill to make everything aligned properly.

Two copies of the part are needed. (You will want to rotate the part as fits your particular CNC, so that the hemisphere faces the cutting surface.) The model has the holes for the rods set at 6mm diameter; your guide rods may vary.

Once you have two copies, insert guide rods, and align. You may then want to do a final-pass milling of the outside layer to make them a nice aligned cube. Then heat, melt, and enjoy!