I watched CoyoteSlinger's recent video on how to ply rope and had sudden inspiration.
https://youtu.be/hW8QGodcdsgI was aware of plying but sometimes you need something to push you in the right direction to notice something very obvious
For an experiment, it worked out very well. I started with 9 stands of jute, folded in half, plyed them together (passing the lengths through the first twist to make the retention loop) then tying a knot and separating the stands into 6 strands of 3 each, braiding those to make the pouch, then plyed to the release knot.
The result is a sling I'm really happy with. Still needs testing but it holds tennis balls easily and, because of the pouch type, rocks of all shapes. This was also quicker than braiding. Would recommend.