Semi flexible? I use 4-40 steel rod in tube. You can get both at Tower. The 4-40 rods are threaded on one end and come in vairous lengths up to 48". I generally solder a 4-40 metal clevis on the servo end and use the threaded end at the control surface.

Yeah, I gave up on golden rod and nyrods years ago. They only get used on throttle, choke, and ignition switches.
Edwin