A shield must have the following parts for it to be considered complete and accepted by customers:
A wooden shield "base" of the correct shape (I'm not sure about rarity), crafted in the wood workbench thing
A boss (1 metal, selected from the radial wheel on the anvil as "shield")
A hand grip (1 metal, selected from the radial wheel on the anvil as "shield")
A "wrist grip" (1 metal, selected from the radial wheel on the anvil as "shield")
A metal frame of the corresponding shape (2 metal, selected from the radial wheel on the anvil as "shield" and cycled through the options to get the correct shape).