I spose the obvious question to start with is how much shotgunning you expect to do and what sort of shotgun your intending to use. The one shot G&P M870's shew through a lot less ammo than the TM, for example. And, of course, what sort of rig your intending to put it on and how much space you have available. The easiest option is just to shove shells into an accessory pouch, though it's far from sexy. If you don't mind taking up large quantities of your rig then you can go with the shell holding panels, or if space is a concern theres always something like a fold-out shell pouch: http://www.sordusa.com/12GA-x-12.html. It's the usual argument of security vs accessability. In my own case I'de prefer to go with a gun mounted shell rack in combination with the shell pouch to refill from that I linked above.
Aside, I've yet to empty 2 shells x 10 shots a shell during a game, due to being consistently outshot and deadified by AEGs at range and pistols up close. And that includes paranoia shots when I couldn't recall if I'd racked it or not after a respawn. Great fun, but unless you're channelling your Inner Operator I wouldn't expect to get through that much ammo.
I just bought the holder from eBay, dirt cheap and has a Velcro cover on the other side should you need it. Shells are pre-fired real steel I re-crimped quickly this afternoon