This is a mold for quickly making snowballs in the shape of a rhombic dodecahedron.
This particular polyhedron has a couple of properties that make it really cool for snowballs:
I've included pre-made STLs in Small (good for quick snowballs to throw), Medium, and Large (good for sculptures), but you can copy and tweak the original design to your heart's content in OnShape.
The models are oriented so that the 45 degree bottom infill is parallel to one pair of sides of the bottom face for a more uniform look, so leave the rotation as is in your slicer.
They print great with no supports.
The pictures are of:
PETG seems to work well to easily release the snowball from the mold, but PLA should work just fine too.
The author hasn't provided the model origin yet.