dealsdual switch personal alarm 2-pack for $13.97

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Add one of @mtm2's airhorns and you're good to go:
http://deals.woot.com/deals/details/4edb1cad-0ec2-489c-9e52-0b5363b9cec3/fiamm-big-air-horn#2

All kidding aside, these seem like a great idea. You can't be too careful these days.