Here are a couple of pictures of my Portable Wargame kit box. It is about 7.5″ square and is 5″ tall (I think).

The flat stuff stays on the bottom while the tray can be easily removed so you can get the pieces setup quickly.
For better or worse, most of the hard parts are done. I will probably add 2 more boards so I can have a 2X3 battle. I don’t think I will have to add any terrain. The only other possibility is, perhaps, more armies for different time periods. Whatever I decide, there is plenty of head space for adding more stuff. I’ll probably have to switch to a different format. Maybe small boxes to fit inside the large box so as to be able to stack the armies “safely.”