I have really seen the light when it comes to hook backed, elastic loop side saddles/shot shell cards for shotguns. They just offer so many more ammo management capabilities with less weight and lower cost.
One of their biggest advantages is the ability to remove and replace them quickly as they become depleted. That sounds good but you still have to work out how you will carry the spares. You have options like sticking a spare or two the loop field on your plate carrier or chest rig(if it has one). You could also carry the cards in AR-15 magazine pouches which works reasonably well. However, if you want a dedicated solution, you might consider something like the new Detachable Shotshell Carrier Pouch MKIII (DSC) from S&J Hardware.
The DSC is a pouch that is purpose built to carry 3 (included) shotgun cards. One of the cards is carried on the front of the pouch where it is always ready. You can think of that one as a speed reload in a sense. The other two are secured inside the pouch. The pouch is closed via a zipper and it features a large pull tab on the top so that it can be ripped open easily with one hand. It is MOLLE compatible with a footprint that is basically the same as a standard M4 magazine pouch.
This looks like a very well thought out solution and a heck of a deal considering it comes with 3 shell cards. Check out the S&J Hardware Detachable Shotshell Carrier Pouch MKIII.