It sounds like your split (pocket) is the wrong size, try to fill it with ducttape or medical tape.
If it is too small, and tape does not work, there is little you can do about it, but if it is too big, you can also make it smaller by sewing the ends together for a little.
But tape is the most likely help.

Willeke

Fine slings, Dravonk! The thick one remembers me to my own first sling made as well from packing strings. I still use it. But now I mainly use flax strings which are normally used for those rolled beef (Rinderroulladen). You can get 100 m rolls of it from "manufactum" quite cheaply. I take 12 of them (5 m each) for a sling that is 1m long from pouch to the end. It is stiff enough to hold the tennis balls. I prefer natural materials and flax is the best I could find so far.

funda iucunda