We know that the control surfaces are bevel which is the problem. Just put the horn on one side, mark it and drill it. The screws should be long enough for the other side to screw into the othe part of the horn, then as you tighten it down, it will conform to the shape with the control surface (aileron or elevator)

If you try to screw it together with the other side being flush on the surface, it may be a problem. Hold it out a ways.