I had an SOE micro rig that I picked up off the E&E once. It was ok, but honestly you can accomplish the same thing with just about any chest rig. At least that is if you're going to slap the 12ga cards on the side of your receiver.
If you're going to actually load from the rig itself then stop reading. If not:
I think everyone and their mom makes 12ga cards now in various capacities. I just got a few from ESSTAC which seem to be everyone's go-tos. They really hold on to that round tightly compared to my older (now discontinued) 3gungear.com cards.
But anyway I've found that you can stick a 6-7rd 12ga card in pretty much anyone's M4 mag pouch. I've got an EMDOM IAP for a rifle mag and it holds a 12ga card well. I also have tried them in ESSTAC KYWI pouches and those work really well albeit maybe a little too much retention.
My current preference (although this picture doesn't reflect it) is to use BFG flapped double-m4 mag pouches:
If I want to run a 12ga card in each pouch they fit, or if it want to do two in a pouch, that fits. I never worry about retention because they're flapped. If I want quicker access, tuck the flap in.
I liked this solution better because it meant I didn't have a $150 chest rig that was mostly for 12ga stuff. The giant general purpose pouch on the SOE Micro rig is great too, but if you're not running an IFAK then it's like, ok, this is a big ole pouch I'm not using. I used to put loose 12ga rounds in there.
Short story: get 12ga cards and stuff them in M4 mag pouches.