The Q-DC, or Quick-Detach Carrier, from Aridus Industries already offers somewhat magazine-like feeding functionality with its ability to quickly swap shell carriers to the side-plate of your shotgun. So, why not offer a magazine-like way to carrier them?
Aridus Industries and Dark Star Gear are collaborating on a new kydex carrier for the Q-DC. The prototypes are being tested (shown above). They appear to be MOLLE compatible and offer retention via a ranger band. These will be available directly from Aridus Industries once they are in full production.
Stay tuned for details.