Forget the plastic or Brass knuckles, you will just end up getting charged with using a weapon. Instead, toughen up your knuckles... do knuckle push ups, at first on carpet, then in time on asphalt and or concrete. Get some grippers, not the cheap ones, but the good steel ones from Ironmind, and turn your hands into tough, crushing paws. In time, you will not feel the need for anything else but your bare hands.