Well, you need one of these:
Latest product from Kris at Kimplacustom (makers of the awesome DEVGRU Glass Breaker) is a GG&G style receiver end plate sling mount; as seen In The Wild.
If the Glass Breaker is anything to go by, this product will be top notch.
It will be available at Toy Soldier and all the usual outlets. Or email Kris: firstname.lastname@example.org
There’s also a version for VFC AEGs.
A note from Kris:
“I designed the middle hole to be a very tight fit. All die-cast M4 receivers are sized differently, due to the tolerances involved.
However it takes less than 5 mins to tweak the hole, with sandpaper wrapped around a pencil.”