I bought a Blue Force Gear Vickers Tactical sling (Larry Vickers from Vickers Tactical is a former Army Special Forces and then Delta Force famous soldier who became a world leading gunsmith, consultant, product developer and now trainer who runs classes around the country people pay TOP DOLLAR for to learn how to shoot a 1911 and AR like he does)... for my new Remington 870 Tactical Magpul.
It is not a shotgun sling per se, but it absolutely works great and I would not replace it for the world, I love it.
I had to buy a swivel or two for my self to make sure the setup I had worked great, but this is so awesome. IT has a quick release/quick adjust so I can carry it across my back/front very tightly and quickly expand it to "operating length" if needed and then quickly tighten it back up for carrying/dangling if I have to.
Anyway that's the one I got, love it to death.
If you were more into a "traditional" type I also recently put a Viking Tactical "padded" sling on my Remington 700, also not a traditional sling but great also, I just like the Blue Force Vickers better for a shotgun where you might be running and gunning, the Rem 700 sling is more of simply a carry strap.