Gimble with 9.5 mm ball

Gimble with a central cup. You add a 3/8" size bearing in the cup for a weight. Any which-way you tilt the outer ring, the weight will always point down. If the bearing is a little loose, add a bit of blu-tack or other sticky stuff to keep it in. This is a remix of http://www.thingiverse.com/thing:100362

I found the original design to be too loose (would not stay assembled at all) so this one has tweaked parameters to print well on Ultimaker 2 in PLA with 0.1 mm layer height. Specifically, d=3.1 and s=1.015 in the SCAD file, which is included here.