As Roo says. Although from a material & construction
viewpoint, one might doubt some of this as meeting
the task --the most stretchy material (nylon), laid (twisted)!
Some mason line is braided.
But it should work, and is available in a variety of bright
colors, which might help (either in variety, or brightness).
(Some pre-stretched polyester is what would be expected!)
If sagging is yet a real problem even with this line,
you might endeavor to set it up with some means
of putting in additional tension, periodically --some
sort of small stick (or --more uniform-- nail) around
which one can turn some line (or tie off with e.g. the
tautline hitch which can be adjusted).
--dl*
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