Definitely make some type of plastic enclosure for the air compressor.
It won't last long out in the elements.
I mounted my air compressor and reserve air tank where the stock jack was in the back and put the air hose up the side into the head liner. The air horn I have is like the Mac Truck horn they sound awesome and need a bit of air pressure to run them, but the added benefit is that I can fill my tires if I have to.
Wolo makes a real nice- REAL reliable air compressor. The kit has a 1.5 gallon reserve tank.
I've got the soleniod switch type...they also sell the chain valve type.
I had a set of Hadley Air Horns- that's what they put in the Macs- the horns were slammin! But the air compressor s#cked! They made warranty replacement too much of a PITA- so I went with the Wolo.
Nothing but praise for the Wolo setup.
When I get around to fabbing a back bumper to match my front I'm going to build a bigger reserve tank into the bumper.
One last time- cover the pump! At least from above and let it pull air from below...