If you are going to use a turnbuckle, you can do the job with just one by putting it on the front of the mast.
You make up one (double length) diamond wire with a t-ball at each end and a ferrul (for swaging) running lose over the whole thing. Like this.



Then you put it on the mast, attach the turnbuckle to the loop at the bottom using a thimble, wind it up tight (with the mast straight - very important), slide the ferrule down and swage it in place.