good job done so far!
I saw on the latest photograph that you have fitted the horn at the lower yoke showing downwards.
That may be a bad idea !
If the suspension works and the front wheel is diving in to its innermost position, the horn and the mudguard may collide.
The mounting position of the horn is originally directly at the frame facing forward, below the lower steering bearing in a hole between the frame downtubes. There are two M8 threads were you can fix the horn holder.
The original holder is a symmetrical, triangular metal piece with rubbers and spacers used for firm fixing to the frame and still some flexibility.