Good call on the cosplay! As far as the emblem... I'd say foam for the base, and model magic for the raised parts. Smooth it out. Then apply a coat of acrylic paint mixed with a heavy gloss gel generously in the color you want. I recommend getting some matte medium to brush on as shadow/texture, to give it that beaten up look. Then apply shelaque if it's not shiny enough, like a thick lacquer. To finish off the metalic look...