I have a box of old horns from old jeeps and trucks, I should test them out and see if there are some that would work for you. Or I could see if my neighbor has a set of semi horns.
The unimog comes with a meep-meep horn - kindof like a scooter. Not very intimidating. I do have air and a nice set of semi horns if I ever get the inclination to make it louder.
Thanks for the input, everyone! I went to the junkyard today and snagged a set from a mid 90's Mercury Grand Marquis. It's a nice, loud, dual-tone horn. Much better than the "friendly" sounding meep-meep.