So, I wanted to make one, that can be opened, so maybe it can be interesting.
First step: Take 50mm (2 inches) polystyrene sheet, cut 2 circles 100mm (4 inches) diameter and cut edges until you reach spherical shape.
Second step: To make texture, use solvent.
Step three: cut inner shape. Build protruding parts with polymer clay or something you used to.
Step four: build form for second half:
To join halves I used 3 magnets:
Paint all black:
Take big and hard brush and paint protruding parts silver:
It’s not finished yet. Inner side must be paint more, but I will do it later.