Number plate holders are a fixture on every Pro's bike. The Rotor number plate holder fits in between the brake bridge and rear brake and includes a nut and bolt to securely hold and display a race number.
Simple to install, holds a plate securely, My only gripe is they no longer come with a self-locking nut. Really guys, for $10 a pop you can no longer afford the "expensive" nut? Ya really oughta include a stainless steel bolt while you're at it as well.
Pros: Makes making your own rather silly
Cons: Cheapo hardware