If you've got room for a compressor...

Hadley air horns
Grover air horns
JC Whitney carries all kinds of kits
Most of these will scare the bejeezus out of people. I'm still trying to figure out which one I want. I'm thinking of getting a train horn around 133 dB...
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Dave Clark Georgetown, Texas MK2A 42Gb - AnoFace - Smoke Lens - Dead Tuner - Sirius Radio on AUX