I used to use an Air Zound. I mounted it at the end of my handlebar so that I could hit it with the heel of my hand. I liked it very much, and think it probably saved my life a time or two, but they don't last very long. Sometimes just putting a new coke bottle on them would fix the problem, but they die after a year or so. They also don't work well in the winter. Currently I use a hand held marine foghorn from REI, just as loud, and works in cold weather. I fasten it on my handlebars with a strip of old inner tube. Maybe $5 total.