Some assembly required; I recommend gluing one of
these to the back.
Metallic Plastic and White Strong & Flexible are available for the extreme budget cosplayer. Metallic Plastic will look enough like metal to get the point across but it will still be obviously plastic. White Strong & Flexible could probably be spray painted to look like a more convincing metal.
Polished Silver available if anyone really, really wants a collectible! But I find Polished Nickel Steel to be just fine for cosplay.