Hello Spotted Bull,
I would suggest you check Jim Webb's book on hunting pouches. Jim has owned 200+ original bags and knows as much about them as anyone alive. Some great drawings in there to work from.
By the way, I would not call that horn a western North Carolina horn. I'd call it a Piedmont banded horn, as these were found in VA, SC, GA and NC. I don't normally associate the banded ones with the mountain types. Great looking horn though. Who made it?
My advice on a bag would be something pretty simple. That way it frames the horn without competing with it. This plain bag was done for Jim Parker's show, and no one would quibble over it matching your horn. If you make something elaborate, you paint yourself more in a "corner".
Just my 2 cents worth. God Bless, Marc