I'm sorry for your loss. I lost my father in November 2010 so I feel your pain...
I think that you should search things from the WW2 era...colors, flags, pictures, maps, whatever...and decorate the box with them.
Unless you wanted to use old pictures to cover the box with and make a small "shrine" of sorts.
The main thing is to make it so that when you look at the box, you think of him. Decorate it in a way so when you see it, you get that warm feeling thinking of him
