A horn should work well. I have used 2" flex tube, birch bark, fiberglass cones, my cupped hands, and even a small traffic cone. Anything to give the call or grunt a more hollow sound. If the horn could stand up to raking on a tree, that would be a plus. I have seen old hunting horns which had a mouth piece, and don't think they would make a good moose call. I should add that if you can mimic the noises that moose make while blowing them like a trumpet, they would work well, but it think a better option is to have a 2" or so opening in the small end to put up to your mouth.