Are you trying to do a "museum quality" restore or are you trying to get a tensioner that works. If the former, than the eBay purchase is probably the way to go. If the latter, then you can make something functional with some 3/8" rod and and a "long" attaching nut or barrel. A true turnbuckle would be nice to be sure but not everyone can make left and right hand threads. I made one for my 9" South Bend by bending 90 degree bends on each of two pieces of 3/8" rod. You can find or make a long nut with right hand threads on each end to connect the two pieces of rod together. (You can make the barrel on your lathe by using a stick to tension the belt while you machine and thread the barrel. Close the shades so nobody sees you using a stick for belt tensioning!) While you cannot adjust the tension with the thing installed as you would with a true turnbuckle, you can pull the 90 degree bend out of the headstock or the countershaft bracket and make length adjustments by turning one of the rods until you get the tension you want. Because of South Bend's clever pulley sizing, the tension will be approximately correct for any of the pulley steps that you pick.
I've since "improved" my setup by making a clevis to thread onto the end of a longer 3/8" rod and the clevis attaches to the countershaft bracket with a pin and hairpin clip. To adjust tension, I just pull the hairpin clip and pin out, turn the clevis as desired, and put the pin and hairpin clip back in place. I don't think I've touched the adjustment since I made the "improved" version. The newer version did, though, have to wait until I got my vertical mill so that I could mill the clearance slot in the clevis.
No rocket science is involved. You just want the drive belt tensioned!
