If after you've cleakoed the skin on, and find you have clearance issues, you will have to do a little fileing.
This will hold true through the remainder of the project as well. There's not lot of this, but as Van's says "make it fit".
Yes, get a small file in there and enlarge it. Make sure you then smooth the opening with some 600 grit, or whatever. Don't make a press fit between the skin and horn.