I was afraid that the thin leather could be damaged by closing and opening the sack and I decided to use metal studs. As a fastening I chose an oval stone.
The stone will be on a leather cord that will hold the stone when the pocket's open. I hoped that in water the leather would extand its size and after sewing it would shrink again around the stone. I kept it in the water for some weeks but it didn't help much, if I'm lucky it'll be ok ;)