Almost too simple to post. I just needed it to replace one from our badminton set that got chewed up by a lawnmower. Took ten minutes to design and ten to print, and three cents' worth of filament. Worked great. Can you imagine finding or making a replacement part that quickly any other way? It took longer for me to post it here than to make it!
Thread and tie as shown, then slide the tensioner up or down the line to take up slack.
The original is pictured in one of the photos. I included the extra "spines" it had; I assume they're for additional stiffness but didn't think about it too much.