I'm not sure if this puzzle is easier or harder than its spherical predecessor; please let me know what you think. The internals have been tweaked now so that the outside does not have any major divots. One nice thing about the ellipsoidal shape is that this puzzle won't roll away.

The OpenSCAD file is parameterized to easily customize this design. One way to make it significantly harder is by reducing the tolerance for a tighter fit. You can also view the assembly as an animation to see an exploded view of the parts.

Print three each of the right and left pieces. The multiply feature in skeinforge is handy for this. These print great with zero infill; I use two extra shells and can print all six pieces in less than 1.5 hours on my TOM.