Simple and cheap answer: Drill a hole and put a sling swivel in.
My method: I put a Troy TRX rail on (only has picatinny on the top). They are light, simply, and configurable. They come with a 4" rail with a QR mount and a couple of 2" rail section to mount wherever you like. They also make a lowpro QR mount that is really slick. I mounted the 4" on the left side and attach the sling via the QR mount. I also have a Troy Ambi QR mount that has replaced my end plate. With this setup, I can detach quickly from the the front mount and reattach at the endplate and it becomes more like a single point, but with more control.
Here is the overall standard setup:
Closeup of the front QR mount:
Front QR detached:
Rear QR:
Sling attached at the rear QR: