I initially used wire rope, but found that the 1/16" aircraft wire would get caught on seams, get stuck between the headboard and sail, and chafe the leech of the sail, so I switched to nylon parachute shroud cord this spring.
The leech of the main is 21" 9" so the main part of the cord is about 20' long, with a thimble at both ends. At the lower end a small block is attached to the thimble, with a 1/8" quick link. The adjustment line is a 1/4" cord that secures to the boom end with a shackle, runs up through the small block at the bottom end of the topping lift, and down to a tube cleat near the boom end on the adjustment line.
Picture 6 & 7 at
http://groups.msn.com/RogerConrad/shoebox.msnw show the setup. click on the pic for a larger image. I also have a close up of the tube cleat, but would have to send that to you directly by e-mail as it is not on my website.