Just finished fitting my new horn; and I've managed to get it and the relay neatly under the R side panel of my M800. I tried a Stebel nautilus (clone) but couldn't get it to fit, so bought a "Ring Hi Note" snail air horn from Halfords, along with a relay, wire, in-line fuse and connectors.
Managed to make a mounting bracket from a screw plate I found in the shed (see photo) and mounted the horn as shown. The relay bolted onto the lower bolt hole (which seems redundant) on the metal frame under the cover (as did the horn mounting bracket).
Ran a 25A wire from battery positive - via a 15A in -line fuse (again from Halfords) - to the relay down beside the battery tray; connects to terminal "30"....... then from the opposite terminal "87" using the same wire to one of the horn's spade terminals; then using the same wire from the other terminal on the horn back to battery negative (earth).
Ran long blue wires (5A) rated from the original stock horn wire connections (just plugged in spade terminals again) along the frame under the tank R side (see photo) and connected them to terminals "85" and "86" on the relay. Tidies things up with cable ties, replaced the side panel and voila! a loud horn which made the missus say "What were you doing in your shed?" from the house.
It all looks very neat and so much better than the stock horn.