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:
