Go to this page and look at the Airzound horn:
http://www.gaerlan.com/bikeparts/acc/light/light.html
I have one of these on each of my bikes and they are very good at getting attention. Just mount the actuator so that you only have to move your thumb over to initiate it...easy on my commuter bike (straight bar) and all of my recumbents, but on the road bike if you're in the drops you'll have to move a hand...if you're riding with your hands on the flats of the bar then it's not a problem. Hope this helps!
-Darren