I am with Mike on the idea of air horns, but dont want the expense of real train horn with compressor etc.
I just discovered that Wolo has just come out with their new model Big Bad Max model 619.
123.5 DB at 320 HZ which can attach to vehicle wiring with 15 -20 amp fuse installed with no additional relay, works then from factory horn button.
$46.45 at Amazon (Wolo model 619)
Here is a good demo on a little smart car that has a good sound clip that compares the old one to the new model 619..not bad!
Now is the time for me to try one out on the Jeep ! My stock horns sound like a Honda.