[email]Optical sensors are cheap. Solenoids are cheap. Springs are cheap. Batteries are cheap.[/email]
Darned Tin Cans aren't cheap: $115 Buck$, plus tax.
If you built your own, you could put them in just about anything you wanted:
Or just set them up to ring a bell, whatever.
Or put them in a mannikin that says, "Ouch!".