Personally, I'd just as soon build it out of plain paper, putty and smooth it, then paint it with a high-gloss black paint (like Tamiya) and then finish it off with Alclad II Chrome shot through a cheap airbrush. It can be carefully brush-painted, but that's too tedious for me. But it does produce a glossy, mirror finish:
I'd prefer that to having to put up with delicate and tempermental silver cardstock and hope I don't get glue marks, fingerprints, or creases in the surface (t'cha, right; I can't even manage that with a 21 inch model. How could I do that with a full-size costume?).