I bought a GG&G Agency rear sling adapter to use with the Magpul sling.
Tactical Rear Sling Mount Adapter for Collapsible Stocks - Stock Side Sling Adapter
It uses a set screw to hold it in place instead of the buffer nut, since we don't have one on our rifles. I used a dremel to grind a notch out of the stock so it would fit. The bottom rail on the tube doesn't go all the way to the lower receiver on a normal AR-15. It tapers off. It works really well and I haven't had any problems with it.
I am thinking of getting one of these. Why did you have to notch out the stock, it fits on the tube correct?
The Blackhawk single point adapter I picked up just spins round and round on me. I can't get it tight enough.