I'm going to resurrect this thread, and bring it back to the original topic.
Joseph, I made three slings using the "trebuchet pouch" that you showed at the start of this topic. I first saw that design in somebody's instructions for building a trebuchet. And I cannot find that trebuchet article anymore, either.
They all work great, although one of them has too small a pocket for anything but golf balls and small rocks. The middle-size one can hold a tennis ball, but not reliably. The big one is presently attached to my heavy-duty wrist-bracelet sling, and it works for anything up to the size of a baseball.
I did find that I must keep the proportions the same, no matter what the overall size. I made one trebuchet pouch that was 7 inches long but only 1.5 inches wide (18 cm by 4 cm). It was basically flat; there was not sufficient width for it to cup the ammo at all.
EDIT: I found it! I found it! I think ... anyway, here is a page showing several pouch designs for trebuchets, including the one you showed a picture of, Joseph. It is here: BackyardArtillary.com/slingsandpouches