u can try mounting it under the seat facing forward i suppose.The biggest problem is the size of aftermarket horns. The FIAMM Freeway Blaster is way too big to fit into the small space behind the oil cooler. I think the Hella Supertone and Stebel models will be too big also.
I tried to install a FIAMM "D" horn, and it was a little too big; it was touching the heat sink attached to the plastic shroud.