I use a length of paracord, with a small loop to tie the braid to, and a larger loop to attach the cannonball. No swivels or terminal tackle whatsoever, the only caveat being you have to use a finned cannonball to prevent line twist.
As for the particulars, the paracord is 2mm diameter (any thicker and the knot sometimes snags on the downrigger pulley), and i use a length of about 7 or 8 feet to start with. Make a simple loop of about 2-4" in one end, and a larger loop of about 1' at the other end. The simple loop knot at the top acts as a stopper. Tie the braided line to the smaller loop the same way you would to a swivel, with a palomar knot. Feed the larger loop through the cannonball eye, and then around the ball, and thats it. You can fine-tune the finished length of cord depending on how you like yours set up. I prefer my cannonball to hang a couple feet below the pulley, with my downrigger booms fully retracted, so my paracord snubber is about 4.5' finished.