And a murky picture of the LHS horn in situ. I turned up a couple of spacers 8mm long and used 40mm cap head screws in the front frame joint:

They're now loud enough that my head's still ringing after using them in the garage......
Be careful with one thing:
The hole of the horn has to be pointing to the ground, so any water that gets in will immediately get out. If you leave the hole pointing horizontally or even worse, upwards, you'll get water in (sometimes even moister is enough), and the horn will stop working until it dries out completely and will eventually rust out. Also, don't make it poiting forward to the water and stones flying out of the front wheel. At the end, snail part of the horn has to stay higher then the hole, just the oposite of the horns on my drawing. When I made the schematics I just drew the horns, without thinking that someone could think that's correct way to orientate horns. If you look at the drawing, you'll clearly see the snail lower then the hole. To install the horn properly, do just the oposite. It's simple, think about anything getting into the horn. If the hole is at the lowest poin and pointing down at some angle, it will go out by itself.
In the photo, it's correctly orientated but it doesn't point down. It needs just to be "turned" on its screw some 20º-30º downwards.