The horn in Pocher Alfa kit is a plastic body and two brass parts that appear to actually be machined. The brass is not finished well though, so into the lathe they go for trimming and polishing.
Also, a handy tool here is a fine file with smooth (non-cutting) sides. I have used this tool a lot to square off "puffy" plastic parts. The smooth side can work right up into a corner without cutting, while the cutting side makes a flat surface. angles closer to 90 degrees improve the look of every part.
The completed horn mounts to the side of the engine block.