Okay, I bought a pack of 5 turnbuckles from our local Dymond store for $9.90 (also got some 2mm nuts to match them for $2 - steel but I can get brass ones from McMastersCarr).
I chucked one of the turnbuckles in my drill press and chopped off some pieces with a Dremel/cutoff wheel. Bad news, I could only get TWO pieces before I ran out of thread. Nonetheless, I now have two reverse threaded "nuts" and a right handed clevis. This took MINUTES to do although my drilling job is rather sloppy.
It would be possible to make the parts using these Aeronaut turnbuckles, which already come with a hex section through the middle part, making nice "nuts".
The clevis shown on the bottom is just under 19mm long overall with the bolt part included, very much in the 1/6th scale realm, and this method allows me to adjust the rigging by turning the wires, just like real life. I don't think the Mick Reeves parts do that, they are all righthand threaded, I would bet.