Put all the boards in the main box, and store all the tokens and figures in a small secondary box. You only need some thing about the size of a double deck box, although I'm loving the suggestion of a tiny metal first aid kit.
With this storage method, we've got enough room for one more expansion.
Sounds similar to what I've done: all boards are stored in the main box and all of the bits are in a thin Plano 3601. Still, I'd prefer one box that holds everything for organization's sake.