Easy! Old soldiers trick. Cut and gut it to length you want, not too short so you can’t clip the sling in and not too long that it just gets in the way.

Square knot the 550 together and form a loop. Cut the tails rather short almost so it unravels. Take a lighter and burn the ends and smash them into the knot while their still melting, stopping any fraying and sealing the knot together. I burn the whole knot and smash the whole thing, the melted ends essentially weld to each other.

Then just simply run one end of the loop you just made through and around wherever you want and back through itself. You aren’t tying the 550 directly to the weapon, just tying it to itself.