found this site:

http://ibiza.comagenius.com/wassermanibiza/sling.html

about Ibizan children making slings in 'gym' class.

they talk about a three plait braid and making the pouch section by simply leaving that section unbraided.

anyone tried this? results?

I've tried it with 3, and I've tried a knotted rope version with 5 (the way I knot the rope, the cradle will have an odd number of strands).  I find that they work quite well as long as you 1.  make sure the missile is firmly seated in the cradle and 2.  accelerate evenly throughout wind-up and release.  Sudden accelerations or an improper seating will send your glandes careening of in unexpected directions.  Of course, I'm throwing lots of oddly shaped, rather small rocks.  Larger, more glandes-shaped rocks work better, and the football shape works better than spheres.