Small diameter line ... 1/8" to 1/4" but just about anything you can easily slip through the eyestraps will do in a pinch.
Length to equal the cable bridle = 31" (not including the knots)
A tight double braid will slide better than a twist type rope.
Tie the main sheet over the rope bridle with a bowline knot. As a matter of fact, a bowline will work for all the knots.
...or use a stainless or bronze snap clip. Either will slide just fine.
Avoid those gateless aluminum carabineer key clips you see
in a fish bowl at the hardware store checkout, they won't hold.
The plain bowline knot is much a more secure choice in that case.
I'm not trying to hijack this thread but, I am using a trigger clip like the one below. Any advantages the snap might have over the trigger or the spring clips?