I've spent quite a bit of time over the last 12 months looking at long line options for tracking & recall training. I've used leather lines, cotton lines, biothene lines and poly cord lines.
I've come to the conclusion that poly lines win hands down when connected to a quality stainless steel snap hook.
Why, it's simple. Poly lines are cheap, they can be washed after use, offer UV protection, resist mildew and rot, can be knotted without risk to their working load integrity and can look pretty good too.
I make them in 6 mm poly cord in lengths from 5 m, 10, 15, 20, 25 and 30 metres in blue/ white and pink/ white combinations. Ask for a price