I have a Reineke leather sling, cost 80 Euros.
It has a stud and loop system so I can pull the sling through the loop and fix it to the stud making it a short sling, just long enough to stretch from the two sling swivels.
Unhitch the sling from the stud and pull and I have a full length sling.
Sounds complicated but isn't really, no loose sling flopping about when using the bi-pod or a fence/post/tree support.