Walther, I cut the leather belt pieces 2,5 cm X 17 cm. That should be 1 " X somewhat less than 7 ". The lenght makes the pieces cross over smootly, and allows tennisballs to fit nicely.
Golfballs and small stones work too, but they do look a bit small and forlorn in the pouch

xxxkid123, that was a nice sling, it is a good thing to be a bit innovative when making a sling

Paleoarts, I must admit I think your bone piece is a little sceary. The way I sling, I would be afraid it would hit an eye or a tooth, according to Murphys law

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timann