One of the bigger problems with the nylon R/C horn is that it's
usually attached to the inboard flap. Then, the flap joiner wire has to receive & transfer the torque to the outboard flap. With the horns shown, the lever applies the torque to the wire, then both flaps. If anything, you'd want to apply the torque to the outboard flap...might eliminate the need to use a 'wart' on the outboard flap, ya know. For the same reason, it would be logical to have the flap horn on the outboard side with any sort of flap horn...the outboard torsion bar being shorter.
Anyway, most those R/C horns are just too flexible, so if you just have to use one, get one for Giant Scale and mount it on the outboard flap. Give it a flex test, if you can. Glass fiber reinforced nylon would be ideal. Cheapies will not be a good choice. I'd be glad to use 1.25" at both flap and elevator. You can use one bolt through both ball ends at the flap. Put the elevator pushrod closest to the fuselage, to reduce the pushrod/flap interference. Adjust the control ratio to suit the plane's weight. Don't fail to experiment with this, you'll learn a lot!
Steve