This is yet another variant of my previous Cello Chair Strap Stop. This one will attach the stopper to two legs so that the cellist will be able to center it and should not slide to either side. For this one, instead of using straps, I used string. For the length adjustment, I created a clip that clamps the string tight, but is very easy to adjust.
You will have to print 2 Leg Huggers, 2 Clips, and 1 stop. The clip should take any string that's around 3 - 5mm, but you could scale it to better suit your needs. Paracord would look nice, but I didn't find any.