Welcome to ArtMetal, Renaldo!

I suggest you do some reading up on mokume gane in the books written by Steve Midgett and others. I think you'll find the answers to most all of your questions in those books.

The box should probably be stainless steel to prevent it scaling or producing oxides. It definitely should be sealed as well as possible. The method that you use to seal it will depend on how you are clamping your stack, the size of your furnace, etc.

The activated charcoal is probably okay, but I'd just use some real wood charcoal - it is there to absorb oxygen and provide a completely reducing atmosphere. Nothing fancy is required.

There are three main factors that influence the success of mokume gane bonding. Time, temperature and pressure. These factors are, in turn, affected by precision of the mating surfaces, cleanliness, absence of oxides and spurious oxygen. So you want the individual pieces of metal to be perfectly flat, absolutely clean and free of oxides or any contamination, and you want them clamped together as tightly as you can possibly get them. Then you need to heat the stack, in the absence of oxygen, to the right temperature and keep it there long enough for the solid state diffusion bonding to take place. The guys who do this professionally use giant 60 to 200-ton presses and vacuum chambers.

Good luck with it! Let us know how it work out for you.

Rich