I sell and install the Viair compressors that go with on-board air or train horns. I wouldn't install a compressor under the seat. Others may have done it, and it may be okay, but that point is pretty low and if the compressor goes under water or gets too much moisture in it, you'll have problems. I've seen a few compressors mounted to the side of the rear racks close to the rear fenders. There's a little room there, but a tank would have to go elsewhere.