I put a really thin section of Weaver rail on the bottom of the tube and use a swivel designed for Picatinny rails. You could also slot the tube for a standard rail hand stop/swivel mount since it doesn't have to be open at the front or particularly long once you have determined where you are holding.
I have a sling on My AR10. It has a wrap around at the front of the hand guard with velcroe to hold it.
Very sturdy sling.
I can not remember the brand of it though.
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