Brodak Manufacturing has about 20 different styles of C/L elevator and flap horns just like that available. They cost about 4 dollars.
I always sandwich those horns into a ply box.
First I figure where the horn wires are going into the wood surface, mark and cut them off. I then make 1/32nd ply faces on the cut off surfaces. Then I make a ply piece to accept the wire, so it has to be slightly wider than the wire diameter. Epoxy together, trim and fit, sand it all level, and you have what we call in C/L Stunt, and horn "clip" for the surface. Probably overkill on an R/C ship because of the slight deflections and enormous airspace available, but that's how I do it.
Chris...