@ametheneko: Ouch. Yeah. That's a tricky spot. I guess if you have a dowel or something that's only like 3 inches long, and then maybe putting in a bit more model magic. You say it cracked from being left in the car? I guess there could be a moisture/heat related thing, so maybe putting more model magic in the crack, letting it dry, and then putting glue on both sides of that little patch piece you made. I've got some model magic floating around, so I'll try out every glue I have and see if any of them stick really well.
OH I JUST HAD AN IDEA. Have you tried Gorilla Glue? It foams up so if you put the pieces together and wrap them with tape like you did before, that might stick better because you'll get more contact. I'm still going to vote putting some sort of support running the length of the horn, even if it's a very short one, because it'll put less stress on the glue and make it easier to get the pieces lined up seamlessly.