Flap horn installation.

I install the flaps onto the wing with the hinges and then center the horn on the flaps using the wing centerline. I draw a line around the horn wire and cut that part away. Then I take a piece of brass or aluminum tube and cut off two pieces that will go over the wire legs that stick into the flaps. I plug the end with a bit of tape to keep epoxy out. Then I lay a strip of 1/16 balsa into each cutout to help center the horn wire. I lay the horn into the slots and shim the flaps out a bit for horn clearance with the straight edge. Then using 5 min. epoxy I slop some in each cutout covering the tube/horn assembly but not so much as you don't want  the epoxy to contact the horn wires. Then another strip of balsa over the slots. When the epoxy cures sand the balsa down to the flap surface. I used 1/64 ply as reinforcement on top and bottom of the horn wire/tube.
  This makes flap installation so much easier. The flaps can be added any time in the finishing process. And they can be removed if need be. Gluing them up on a very flat surface assures that they are in alignment.
  If you like to taper your flaps sand them down after installing the flap horn in this case.