Glad the 306's fit how you want them to. I've discovered a little more about the Original Pack too, in case you ordered one after all. I just popped mine apart, and it's also extremely simple to mod so you can fit an assembled cig in the proper hole even with a 510 carto on it.
The foam insert is just glued around the sides to keep it inside the pack, a thin knife blade slides around it easily and since the inside plastic of the pack is smooth, the cheap superglue or whatever they use doesn't hold to it too well. The foam pulls out very easily after this. I slipped the knife around the edges of the plastic "ledge" underneath to see if it came out because there aren't any seams on the sides of the pack to pop it in two. It turns out that ledge is part of a larger insert that houses the battery and charging circuit too. With a little work the whole thing popped out of the outer shell. You won't need to do that though. Since the whole thing below the foam is plastic, all you have to do is sand or file the hole to make it a little wider and a 510 carto slides in perfectly. I just took some sandpaper and wrapped it around a pencil. Once you're done, just put a little glue around the walls again and replace the foam insert. It looks like you can then fit an assembled cig with 510 carto, spar batt, and at least 3 more 510's and two blu or 306 cartos in at once. The foam compresses with the 510s in the carto spots, and I don't have enough open to try fitting 5, but it's probably doable.
I'll take some pics before I reassemble the whole thing permanently if you'd like to see, but I want to play with the charging port. It's still got a spring loaded center pin like the starter pack, so I think if I file down the threads a little, and add a spacer to the lid above the charging hole, it should eliminate the need to screw the bat down for charging. If only it were a smart charger too. I'm betting I can swap the circuitry from a starter pack into the orig. pack though, just have to get my hands on one cheap enough to experiment with.