I have been using different weights of coated stainless steel fishing leaders. Today I made one up using 20 LB. coated cable. It slides right on through the holes on my 2-56 rigging couplers. I'm using 2-56 on this system because the plane is only A 60 size Extra. With bigger planes I go to 4-40 but the system is made exactly the same. I'm running the wire through some small plastic tubes that are normaly used as push rods. You don't need the tubes but it sure makes life easier if you install them during the build.
I have this wire leader in different weights and thickness, bought at any fishing supply shop.