TheJackinati wrote on Apr 4
th, 2019 at 4:44am:
Jute should work fine.
Sissal would also work, though you will have to cover the fingerloop unless you like blisters on your fingers.
Hemp would probably be the best, as it is strong yet soft on the hands.
Nettle or bamboo fibres should yield a good sling.
Cotton will work so long as you rub the individual yarns in beeswax before starting, otherwise cotton twine can 'fuzz' up.
To be honest though, you should just do some experimenting. That is probably what everyone else does around here in order to find what works for them.
This
Edit: for the record Jute is my favorite natural fiber to work with. It's easy on the hands, easy to work with, cheap as dirt, reasonably durable, and makes a good looking sling. I do like the others mentioned also. Cotton butcher string makes a pretty nice sling.