Gömböc is a shape that if on a flat surface rotate to one point to be stable a stable equilibrium.
See wikipedia - Gomboc or Gömböc

The one I created is hollow to fill with filler best is concrete plaster mix. (There is a hole at bottom)
Then heet it up to 130 c and peal the outside off.
The cast hole,sand it down to a flat service.
Make it not too small about 15 cm will do.

The shape in Pla only is too light in weight to work well.