You should have a look at the "Hopson Sling."
It does what you're looking for and more. It is a 'tactical sling' hanging down in front of your body for transitions, etc., but it can also be configured to function as a conventional carry strap for African and American carry. It may also be used in 'backpack' carry mode.
It comes complete with Uncle Mike's QD swivels.
The sling sells for $40.
You may contact Cecil Hopson direct.
He is the inventor, patent holder, and manufacturor.