The M2 Tactical is cursed with pretty much no options for a sling loop. It seems like the M1 got all the glory.
I ordered a Benelli M4 loop attachment from Mesa Tactical and started working. I removed the stock and drilled a hole straight through so I could bolt the loop down. It came with all the hardware I needed. After everything was cinched down, I cut off the excess bolt and filed it a bit. I also hit it with a cold bluing.
There you have it.
Hanging from a Gear Sector ASP sling. I had to push the shotgun forward to keep my toes out of the picture.