I've always used a regular 1 1/4" military leather sling. If you adjust it correctly, and use the leather slides, you don't have to disconnect the rear swivel. In the field, or if you just plain don't feel like screwing with it, you can just wrap your arm in it once, and you have a pretty firm platform.
I recently picked up a rubber like sling, but haven't tried it yet. It's made from some kind of super wizzy gizzy rubberish material that isn't ever supposed to crack, and will allegedly support the weight of a whole aircraft carrier group...or something like that. :grin: There doesn't seem to be much give to it, and it seems to provide a pretty solid platform. But like I said, I haven't had a chance to actually shoot with it yet. It's made more for carrying the rifle, but has adjustments to use it to shoot with too. I'll bet it doesn't get nearly as heavy when wet as my leather military types do.