[Graham]

The problem is that if you make the outer casing of wood or plastic, you greatly add weight and therefore cost. 
Our current box with all the cars and internal boxes weights less than 10kg
What I could do with is either make the current foam board boxes better so that I can use masking tape to seal the lid on and perhaps with additional box covers that zip over or alternatively something like plastic trays with clip on lids as each of the internal boxes. But dimensional these plastic boxes would have to be exactly right dimension to fit into our 45x45x50 outer casing.
Perhaps the internal boxes could be tailor made by a 3d printer ??