Idea (please correct me if this would not work for some reason? I officially have not tried it, just thinking)
You could leave the outside horn piece somewhat roughened and squared (not rasped/filed to final form for a few mm) and just clamp it tightly in a vice at a convenient angle? Then once you have the hole drilled roughly how you want it, check the fit, use a rat tail rasp or something else finer to get the shape just right to fit, and Then when the bow tip hole is shaped to your liking, you take the horn out of the vice and rasp/file to shape.