I know lots of us are sleeving our cards, and if you are sleeving your cards I know you'll eventually discover the plastic insert included with the game (as awesome as it is) won't hold your cards. It was deck 2 for me.
So I measured, calculated, and built a new insert from foam board. I posted instructions for you to do the same here in case anyone else wanted to do that same: http://boardgamegeek.com/filepage/99044/constructing-a-foam-board-insert
If I'm right, it should hold all the cards for all the decks all the way to deck 6. It also has a slot to store extra things. I put some pens, standees, and dice in mine.
Anyway, its there for you if you want it. Thanks.