Here's a sling David and I made.
It goes across both shoulders and attaches to the rear of your rig. Its completely adjustable and ambidextrous.
It has a disconnect with a fastex buckle that floats on the sling and automatically centers itself when there is weight on it.
It attaches on to the back with long MALICE clips and has webbing on the front that you can place pouches over. The adjustment is on the right side in the back.
The disconnect attaches to the weapon with a QD sling swivel, but could be made with whatever type of attachment is necessary.
I like it quite a bit so far. It seems to distribute the weapon weight over both shoulders equally and since it attaches on the back, it might offset some of the weight of a hydration carrier or whatever you might have on your back.
I suppose this setup might even work attached to a belt, but I suspect it would not be as useful and comfortable as it is on a torso rig.