I have two X NAUTILUS air horns, in place of the original, even used the original horn brackets by bending end piece through 90 degrees, these things are seriously loud, they even fit(just) with RS500 cooler there, then just a simple bit of wire from bracket to horn for earth plug in live each side job done.
The nautilus horns are compact two in one air horns, so a high and low each side.
tabetha