If you don't want to remove the castle nut there are a few bolt-on solutions. My favorite is the Tango Down PR-4.
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This piece weighs only 2.4 ounces, straps around the buffer tube and the castle nut, and provides three QD sockets: left side, rear, and right side. I solicited some feedback on the PR-4 before purchasing one and all respondents reported a solid lock on the buffer tube and flawless function from installation forward. One person chimed in to say that his duty weapon has been hanging from a PR-4 for more than four years and has never budged.
I have one mounted to my M&P15T and use it for a quasi single-point sling (a sling worn like a single-point but with QD swivels mounted on both the left and right side of the PR-4:
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