I drilled out the stock where it looks like should have a qd cup installed from the factory, (it should) using a 1/2" bit, drill all the way through the first wall, but not the second one.
Used a Magpul (MAG333-Type1) mount kit, you will have to tap the cup in using a poly or rubber hammer, the fit is tight.
Then with a 3/16" bit use the cup as a guide and drill the hole for the screw through the other wall. I also used a delryn spacer inside the stock to keep the screw from pulling the qd cup into the stock or breaking the wall on the screw side.
Worked well.