Ahhh, what you need is a Kahlenberg horn then. One of these baby's will make traffic come to a screeching halt while everyone waits for the QEII to clear the intersection.
Hear it!
Unfortunately they don't have a linkable picture for one of their, umm, beefier horn assemblies (about 3 feet in length!) but here's a sample: Hear the T4-A!
But if you've got 4 feet to spare somewhere, and don't want to mess around with compressors and air tanks, then this is the uber electric horn:
Hear it!
But if you still want the "big" sound in a more compact package, plus the added benefit of people going "What the ... ???" when looking under the hood, then how about this:
Don't look like much? Listen to it! Still need the compressor for that bad boy. ;eekg;