What about something like this 12gm setup that I'm going to try some day soon....
clicky to make biggy.
Yes it's a little odd being a power feed but the angle actually lends itself to loading the 10 round tubes into. The grey PVC adapter was made from some plastic conduit and I'll likely make a new black one before I go nuts with it.
The real key is the vertical adapter and 12gm holder that makes it easier to change out the CO2 cartidges easily. It's out where I can screw it on and off easily and that's a big concern when you only get around 20 good shots with a 12.