I haven't installed yet but I am in the process of getting organized to install an Airlift 7200 wireless system to control the airbags on my truck. One of the things I came across while searching for install ideas is that a lot of sellers do not recommend putting the compressor in the engine bay. Apparently over time the heat may have adverse effects on the compressor functionality. That being said I have also read where a lot of people have put them in the engine bay and stated that they have not had any issues. YMMV.
One thing I did see online that I liked and am copying is where a guy took an ammo can and mounted his compressor on the inside of the lid, cut breather holes in the floor (now ceiling) of the can and then mounted the whole thing upside down to the frame rails of his truck. The idea being that the ammo can provides protection against debris, salt and water. His thought was that if he ever needed to access the compressor he could open the lid and the compressor would swing down from the underside.
In any case that's where I am at the moment, trying to shoe horn things into a 50 cal. ammo can. Whenever it stops raining I'll climb up under the truck and figure out a decent spot to mount the thing. One idea I had was to purchase a waterproof computer fan, mount it over the ventilation hole and wire it up so that it comes on when the compressor comes on. (Probably way over kill, and I likely won't do it.....but it seemed like a need thought at the time)