The add-on horn came and I installed it. It appears to be a 3-D printed part. I suppose that a mold could have been made from a 3-D master, but it has the striations typical of a printed part.
My snow target board is rapidly losing snow in the rain today, so I cleaned off all the snow and will start over. I am watching for stability. I don't know if I disturbed the mounting arm and need to re-calibrate yet. (this involves changing a couple of constants in a script)
It fit well and looks level and even.
Greg H.