I've used a few diffrent ones. Honestly one of the ones that worked the best for me was a rigged issue service sling like how I have it setup below.
550 Cord around where the delta ring normally is and the other end strapped around the buttstock. Works good for me and is free since I can get those slings.
I also have used 3 point slings which work okay but the extra sling piece running across the rifle can get fouled in your gear if that is a concern for you or not.
I have yet to use any single point slings but I hear good things about those for a lot of people it's personal prefrence. The sling I rigged above works well for me and is very cheap.
Here is a sling I found on CTD website that will probably work for you.
This one will work if you have a 20" model this may not work for the shorter ones. Too much material.
Here is one that will probably work on the shorter model ARs
Here is their tactical sling page