You can make the belts out of pleather. You're better of using garment-weight pleather, upholstry weight is more difficult to sew. You can use fusible interfacing to stiffen it, or glue it over craft foam if you want it to look thicker. The metal parts (buckle, cap) can be scavenged off a thrift-store belt.
For the collar, use interfacing. Soft craft felt works fine if you don't want to have to buy a whole yard of interfacing; put it between the outside and lining of the collar and topstitch all three together around the edge. The white bias tape will keep it from showing.
For the metal piece, sculpt it out of whatever smooth, paintable material you'd like (craft foam, Sculpey) and use jumbo sew-on snaps to attach it. You can hotglue/superglue them to the metal piece, and sew them onto the collar. People use snaps like this to attach armor, they last longer than velcro.
Fast. Cheap. Accurate. Pick two.
Current Project: ALL THE HATS (Oriko Mikuni) (Princess of the Crystal) (Shikieiki Yamaxanadu) | Upcoming Cons: Otakon 2013