Step1

Print the Mold and lube it with vegetable oil on all insides.

Step2

Use "DIY Knetbeton" (fiber enforced modelling concrete) to fill the mold.
Optional: place some wire for reinforcement.
Let it "dry" at least 24h in the mold (better 48h) - dont touch, no earthquakes...

Step3:

Break apart the outer shells except the "stamp bottom". Dont try to pull the bottom part off the concrete piece, since it will damage the surface a lot.
Instead melt it off with hot air gun and/or place the the part into your next BBQ or campfire to burn off the remaining mold parts :D

Be careful with the outer arms (mine broke, but I've added some copper bonding which will get natural patina over time).
Some smaller pieces will break off but that makes the whole think more look like an ancient artifact...