After my initial not so successful attempts to create a snapping sphere I had a chance discovery when I tried to put a ball inside the two sphere halves and noticed that although the two parts of the sphere don't hold onto each other they both snap to the slightly smaller ball in the middle...So I went back to the original symmetric Demi-Sphere http://www.thingiverse.com/thing:3068 for the shell but added an inner sphere (without the hard to print top and bottom tips) to hold the two demi-sphere parts together. Snaps like a charm.

print 2 x metaball4_shell.stl and 1 x metaball4_inner.stl

there is also a slitted version of the inner part...can't decide which one I like better...