Use your body as a stabilizing base - like a position shooter - instead of allowing it to hang off your extended arms working against you. Your shooting hand and wrist against your chest is now your stock weld. Put the sights up high enough you don't tilt your head over - it puts the tube over the shoulder, which is exactly how a lot of others shoot the AR with a stock - with just the toe in the socket.
Put the other arm down against your chest and grab the front barrel shroud (!) to stabilize that end. With both elbows against the chest it's not going to weave around like a pistol hanging out extended. Add sling as you wish.
It's no different than position shooting, use your body to support it. Ergonomic, right?
Hoping others will chime in with how the human body can support the gun, but all said and done we have hands, forearms, elbows like everyone else and it's all we get to use. Stack them up, there's not too many ways to do it.