For standing, where you can't really lock you elbow against anything the best sling is probably a two point. You use a hasty sling method where you wrap the sling around your front forearm, drawing it tight against your chest as it is connected to the butt stock.
For kneeling or sitting, you can use the same method but a loop style sling is better. Here the sling is only connected to the forend and forms a loop that you slide up your arm and cinch in place above your elbow.
The old standard M1907 US leather military sling does this and can be used looped or hasty. I use it on my hunting rifles too, as well as target.
Basically, any sling material can work as long as it is not elastic. I prefer leather but synthetic is fine too I believe. You want 1 inch wide or better to spread the pressure out on your arm.