I use the 5.11 Select Carry sling bag meant for the MP5 (linked above, though I can't believe they still cost that much). It works great. Of course the gun came with the 30 rounder bolt hold open mags. It fits with one inserted, but it's tight. Instead I keep a 20 rounder in it. I also have a double mag holder strapped to the pals webbing.
Sometimes I practice swinging the bag around to the front, pulling the gun and throwing the sling over my head. It's a great little gun.
In a couple weeks, my gun club is having an Action Shoot where we allow carbines in the Shoot House. (normally rifle calibers are not allowed in the shoot house). I can't freaking wait.
5.11 isn't exactly a discrete pack for gun people, but I think it works. I don't wear the fanny pack part on the bag. I've worn it to Law Enforcement conventions, Gun shows, and the Outdoor show as an actual sling pack for all the handouts and swag and nobody accused me of having a sub-machinegun.
I can comfortably keep 1 mag in the gun, two in the double holder and two more in the smaller zipper pocket.