Brass is a hard metal, and you might hurt someone! Use these wooden knuckles instead.
These should be considered "art" and in no way should they be used as a weapon.
Use a laser cutter to cut out of wood, acrylic, cardboard, or whatever.
I had to use two layers of 3mm Baltic Birch plywood and glue them together, YMMV depending on the power of your laser cutter.