Idea 1: How about buy a blank standard game box and decorate to hold the rules, charts, and pieces? Store the Stalingrad map in the OST box, where it now fits.
It is definitely easier to buy a standard size box than an OST size box.
Idea 2: see if flying pig will sell you an empty OST box.
If flying pig sold me an extra box, that would be awesome. That's the best idea yet.
Your other idea would work well also. I guess I have just been thinking that the expansion should have it's own box. If it's all on the table like mine is now it doesn't matter. But if you went somewhere you would have the option of taking/not taking the expansion. If you combine everything into a box and tray or a big box and smaller box, then you have to take both.
The game is just so big it creates these wonderful problems. And it's only going to get worse when vol. II comes out