The plate which usually goes on the bottom is permanently mounted on the upper side of the tube with zip ties. The horn can then be removed separately.
Read Alexander's description below:
The obvious problem of thicker downtube was solved with zip ties but it turned out to be tricky as the downtube was thicker and ovalish at the front and then became less so in the middle. It caused even tight mounting to become loose as due to constant shakes and bumps the horn "creeped" lower and lower. I am not an "urban warrior" type of rider but occasional drops from pavement and small potholes are unavoidable.
So far the best results were to mount the horn bracket "inwards" and pass a lot of zip ties between it and the horn before tightening the screws. Also I placed a bit of rubber where zip ties touch the downtube. After tightening everything the horn sits pretty well, completely unmovable by hand. This is still testing setup, I plan to add more rubber and try metal ties. The horn seems to remain dismountable by unscrewing the bracket without cutting ties but I'll test it further.
Also I am still searching the perfect spot for the button. I don't like it directly on the grip as it reduces "grippable" surface and might cause accidental honks.
Just wanted to share this. Sorry for not so clean bike - Moscow winter is not pretty :)
We have to note that his bike is pretty decked out with that speaker system.
To complete the overview, here is where Alex keeps the button.
Good luck with your fitting and let us know if you have questions info at loudbicycle.com