There are dozens of options for slings. You can install a Quick detach stud on the rear stock and get a magazine cap with a quick detach stuf. This allows you to use a 2 point sling. This is the easiest option and best for what you intend to use the weapon for. Avoid the sling plates and single point stuff as that is not what you want for a weapon you are taking camping and hiking.
would have all those items. You want to get an 870 12ga (I know your weapon is a 12 as you put a 12ga barrel on it) magzine cap with quick detach stud and you want a srew in quick detach stuf to install in the read stock. You can get an inexpensive two point sling with quick detach swivels from Wal-mart or any where gun stuff is sold. In fact, if there is a wal-mart, Dick's, Sports Authority or any of those type of places, check there before you go on line. The guy at the counter can help you with the items you need.
Just remmember to take the sling off when the weaopn is serving in the home as an HD shotgun.