For the Project 590A1 shotgun, I've added a patch of industrial Velcro to the left side of the receiver and attached an Esstac Shotgun card. When empty, the shotgun card can be peeled off and a new one slapped in its place.
The elastic shell loops on the Esstac card are quite tight, which means they have great retention and a firm grip on the shell. But, that also means they can be a bit difficult to load the shells onto the Shotgun case. and a bit slow to retrieve. They should loosen up well with use, though. I doubt they will be as fast as a kydex type material, but they are pretty good.
Running a shotgun is a lot about managing your ammunition and regularly topping off. The shotshell cards are excellent for this--fire one, load one; fire two, load two.When the card is empty, slap on a new one and continue.
The shotgun cards are $15 a pop, so you can pick up a few for what you'd pay for a decent conventional side saddle. And install takes all of about 5 minutes--cut Velcro, add to receiver and then let the adhesive set.
The cards will also fit in most magazine pouches--I've run mine in my Costa Leg Rig, where they work quite well. The Leg Rig's mag pouches, at least, oddly had some loop strips on the inside, which needed to be removed/covered, else the shotgun card's Velcro hook would latch on and make drawing the shotgun card quite difficult.
Overall, I'm pretty happy with the system. If you're planning on running a sidesaddle for reloads, this is probably the best way to do so.
The Shotgun cards are available from SKD Tactical or direct from Esstac. Industrial strength velcro can be found at your local hardware store.