Looked into a Johnson RediMag?
Will hold a spare magazine ready until the magazine release button is pushed, when pushed the empty will fall to the ground and the spare magazine into your hand. The RediMag will hold 5, 10, 20, 30, 40 round magazines with no problem. Also gives you the ability to use standard 3 mag pouches without running into fits due to having magazines banded together.
Down sides include having to bolt the thing to your rifle though it's not permanent, it's just not a very quick install or uninstall. The spare magazines held in the RediMag can be a little loud from having a sort of "slap" sound made between the looser tolerances of the redimag and the magazine(compared to the tighter magazine well holding the mag). And some people don't like the way a spare magazine might throw off the balance of the rifle, though it's not all that different if you're banding magazines to one another. Last, some don't like how it makes it a little bit harder to get to the bolt release.
To get a gander take a look at the following link for the one I keep on my 14.5in. bushy. I have enough of these to mount to all 3 of my AR15s if I see the need to do it, as it stands now I only have one rifle that keeps one mounted to it at all times.
[url]http://home.bak.rr.com/varmintcong/14.5in/bushmastershorty.html[/url]
If you get a RediMag, get the A1 model as it has a button added to it to make it much easier to manually lock the bolt open.
The things aren't terribly cheap though, usually about 45-55 bucks.