i found a way to improve a one-string sling design, if you don't have a good cord available.
my friend has lost his sling, and all he could find was 3 m. of badly twisted, uneven hemp rope.
he made a one-string sling design, but the rope was to thin, whole sling too flimsy, so most of his stones dropped out during the wind-up.
after seeing it wasn't usable we cut 2 short pieces of spare rope (2-3 inch each), and bind them vertically to each of the three strings that made the pouch. (we bind them so they divided the pouch in three equidistant parts)
we used the most common knot, the one wich you start to tie your shoe laces.
/just try to put the bulging part of the knot on the outer side of the pouch./
it would be better if we had thicker cords, and thinner stabilizators, but it functioned great anyway. not one rock got ot of the pouch prematureley, except when it was the operator's fault.
he could sling pebbles soze of a thumb, to those that fit in your fist niceley, because pouch was now more rigidly held in place than we anticipated.
strings were held equaly divided (both vertically and horinzotaly looking), firmly in place, and abe to cast ammo of various size (with wich normal one-string slings have some difficulty, mostly depending on the texture of the cord, and it's thickness).
if all you have is a rope, and you prefer knitting to the most simple version of the sling known to me (wich is i am still entranced by it) because of better consistency, or don't trust in one-string to hold it's ammo, here is your solution (and possible saviour of the sling-trip/day).
tell me if you comprehended what i tried to articulate in the 1:10 in the morning, after a dull day