Can be crafted in the Aetherium Forge during the quest 'Lost to the Ages'.
Or alternatively, when Aetherium Armor and Weapons Compilation is installed, all three Aetherium items can be crafted, however, apart from the quest relevant item they will use a different recipe.
You need to have the original in your inventory for the replica recipe to be available.
Replica created at the Replica Station:
Or if Aetherium Armor and Weapons Compilation is installed: