materials used to make slings:
ducttape - actually found one I made years ago when recently clearing my desk: pouch and whole sling
medical tape - pouch
plastic shopping bags: shredded and braided.
leather - both fully leather braided (got 2, I think) and just for pouch usage. My first sling had leather shoelace cords and I think a single layer denim pouch - not recommended unless it's braided leather - they snapped in use.
Denim - both single and double
Alligator hide - pouch
3D printed Flexible Polylactic Acid - currently that one's just me
used for Pouches and finger loops.
All manner of natural fibres and animal hair & wool
manmade cord of all sizes and descriptions
And that lot's just the ones I've got or have made.
You can make a sling out of just about anything.