These gentlemen have great ideas. The ply-horn that mark explained is what I use on all my ringmasters, and even the 800 square RM.
The combat horns are nice, and super easy to locate. There's the cheap and easy way, with a 4-40 or 6-32 bolt and two tee nuts, and the dubro plastic connector for ailerons or whatever.
Then there's the adjustable commercial type of post type horn. They're used often on giant scale 3d models also.