Hi all sorry about the spelling, the one that sags is a loose cotton covered elastic, which is quite thin. I have used (supplied in kit, about 1 mm dia.) some black woven covered elastic and this needs to be stretched to stop sagging. One of my yachts (kit review) was on front page of MMI with sagging stanchion cord, most embarrassing!
There is a white / clear solid elastic round section of various diameters available and this looks fine. I have not seen it in a thin enough gauge for rigging line.
Perhaps fishing line or similar could be tensioned out of sight?
regards to all, Roy