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You could just get some metal from Hobby Lobby and polish it up to a high shine so it is like a mirror. They sell small sheets of tin, brass, aluminum there along with tubing. Might work for what you are trying to accomplish.
Let a billion souls burn in death than for one soul to bend knee to a false Emperor.....
"I am the punishment of God, had you not committed great sin, God would not have sent a punishment like me upon you"
I've seen people use aluminium foil from their kitchen or aluminium tape for small scale mirrors, maybe you could put a thin sheet of clear over it to make it more reflective. It looks fine for small mirrors but for something bigger you probably want something else.
Let a billion souls burn in death than for one soul to bend knee to a false Emperor.....
I'd test it on some bits of sprue first to make sure the paint's compatible with the plastic, but it should be 
