I prefer the rear sling mount for a 2-point sling on my AR-15 to be as close to the rear of the receiver as possible. This usually involves replacing the end plate with one that is designed to mount a sling. However, there are times when the end plate is difficult to remove (or you are not permitted to remove it) and a bolt-on approach is preferred. A number of options exist for those times that a bolt-on solution is needed, but I have never seen one quite as compact and streamlined as the Mission Adapted Design (MAD) Hook Up.
The MAD Hook Up is a sling mount that clamps onto the receiver extension (buffer tube) on the rear of the lower receiver. Its trim design keeps it out of the way of your hands when manipulating the charging handle and allows for the use of a number of different attachment methods like HK style snap hooks, ITW MASH Hooks, and threading the sling directly to the MAD Hook Up. The designers took care to break the sharp edges so that the MAD Hook Up would not chew slings to bits.
If you haven’t tried a 2-point sling mounted in this location, I strongly encourage you to experiment with it. It allows a tremendous range of motion and smoother transitions from shoulder to shoulder. Of course, the MAD Hook Up can also be used for single point slings. Check out the MAD Hook Up at MissionAdaptedDesign.com.