When I installed the horn on my rib, I wanted it to be as loud as one of those hendheld air types, so ended up popping into motorworld, and for something like £12, got myself a proper job which fits perfectly on the a-frame.
See left of pic for horn - ignore the zip ties on right with loose wires - that's the bullet cam.
I ran a new 20amp cable for the thing since the compressor is a bit heavy on current, and pointed the funnel downwards to avoid salt going straight into the front and wrecking it. The motor sits inside an ip rated box thus completely protecting it from the elements.
The box did come with 2 horns, but 2 would have been slightly OTT I thought