In the first picture, the C-shaped clamping piece looks to be a bit far recessed into that half of the housing.
Mine needed to be very slightly proud (0.5mm?) so that when the two housing halves are screwed together, they don't quite meet (leaving a gap that would fit a piece of paper), so that the clamp bites the steering column. Otherwise the two halves meet tightly and the clamp is still sloppy on the column.
The clamping piece sits in slots in the side of the housing, which wear, but you could build up with body filler or araldyte to stop the clamp sliding too far in.
Also, insulation is very important so you don't inadvertently short anything, especially the horn.
HTH