If you can find an old electronic police or fire siren -- they almost always come with a "Horn" feature (watch the show COPS or listen the next time a similar vehicle passes and you'll hear what I mean).
I had one wired into the horn button when I ran volunteer Fire/EMS. Basically it consisted of a control unit (including a knob for selecting different siren tones), wiring (you could setup an easy push-button, or wire up a horn switch relay if the unit is equipped), and a speaker (I used a 100-Watt variety with a large bell housing). It's been a while, but I bet the old stuff can go pretty cheap on eBay. Of course, you'd have to disable the sirens or expect some trouble.
As far as effectiveness, it was loud and different enough to get the attention you needed. Not sure about the laws of its use in CA.
RH77