I've got a 870 that I use for 3-gun matches and keep in the closet for HD. I've got a Choate, I think, side saddle on it that holds six rounds, and a gun show buttstock cuff that holds five rounds. And the buttstock is a speed feed stock, so with everything I've got 7+1, +6, +5, and +4 all on the gun :) It gets a little heavy. http://gallery.gandalf23.com/v/gunz/gunpics/remington870.jpg.html?g2_imageViewsIndex=2
It's got an 18.5 inch barrel, so the magazine extension sticks out a little :)
No pics, sorry, but at previous matches I've used a Blackhawk STRIKE SAW pouch which has a snap in thing with a slit in it that lets you reach into the pouch and get shells out, but keeps them inside and not flying around as you run. there do not seem to be pictures of it on Blackhawk's site, but I found this via google: http://www.polimil.co.uk/acatalog/37CL28MC.png
. I added a shoulder strap to it and it works. But not great.
Recently I, too, wanted to add something that would hold shotgun shells to my existing AR15 rig, so I looked around and got some of the Esstac cards. I have not yet removed the side saddle from my 870, so I haven't really tried them out yet, but they seem a good idea. I use surplus USGI two mag woodland camo AR15 pouches
on my 3 gun rig, so the cards will be protected from the mud and whatnot at the matches. I'm not sure how well the velcro will hold out, especially the piece on the shotgun. I imagine it'll get full of dirt and debris rather quickly, but I am not sure.
I also ordered a few of the Original SOE 12 gauge pull out trays. http://originalsoegear.com/12tray.html
I figured I could use them with the existing setup, and there is no velcro to worry about. The trays hold 16 rounds of 12 gauge each and fit into the USGI woodland camo two mag pouches. The idea is that you tie it with 550 cord to the drain hole then pull the tray out of the pouch, reload from one side, then filp it over to reload from the other when you need that. The 550 cord keeps it from getting lost and keeps it in position, sorta. This seems to work out great, which is why I have not tried out the cards. I gave them to my brother in law to use on my 1100 which he has borrowed.
The trays are kinda expensive at $25 a pop, but could be easily made. I ordered about $30 worth of stuff from ParaGear to make a bunch. IIRC I bought some three inch wide webbing, and some 1.5 inch wide elastic, and some thread. It's still sitting around in the box, as my wife got rid of her sewing machine when we got married since she never used it, so they've not yet been assembled into trays, but one of these days I'll get motivated and take the stuff to my aunts and use her sewing machine. Although I just traded for a Saiga 12 gauge, so I may need to change to something entirely different. :)