Okay! Here are my thoughts for making the decorative pieces to attach to the belt. You'll definitely want something flexible like craft foam. It comes in a lot of colors, but if you wanted to make it a metallic/silver color you could buy a small bottle or two of Acrylic paint (I personally recommend FolkArt for metallic paint, but DecoArt or Americana work just as well). If you are concerned about paint rubbing off onto clothes or other parts of the costume, you could purchase a can of acrylic matte (or gloss, depending on how shiny you want the pieces to look when done) sealant/finish. For the metal circles, you could buy something similar to those round plastic glue-on gems. I've seen glue-on silver circles before, but I'm not sure what they are called. I think they are usually next to the craft foam and/or the gems or feather boas in stores. If you are planning to glue the pieces on, you will want a strong, glue that bonds to fabric/foam and dries flexible. The chemicals in some glues are not kind to foam, so it is always a good idea to read reviews online beforehand.
Not sure how thick the belts are, but if you're looking for a more permanent option you could always go the sewing route and purchase a metallic non-stretch fabric, fold/iron the rough edges under, quickly tack them down with a few basting stitches by hand, and then top stitch the pieces onto the belt. You would need polyester thread for this since it offers more stretch to the seams, and if you use a sewing machine you would need to buy a pack of heavy duty needles (assuming the belt isn't too thick for the machine).