If the problem were to just make a box that itself did not burn in an housefire, that seems trivial and somewhat useless - 100 cm cube of brick that holds 0 (pick your volumetric units) contents
If the goal is to protect the contents from a fire you have to understand that many items one wants to protect would deterioriate with heat.
Even the best insulation (probably aerogel) can only buy you time. If it takes half an hour for the inside of my box to reach 100 degrees in a 500 degree fire, I just need to keep the fire on it longer and eventually I can get the contents to hit pretty close to the 500 degrees.
If you use a more active solution such as a compound that will absorb heat, possibly releasing a relatively inert gas that will pressurize the box so that flames flow away rather than in, cost will go up and it will still only last as long as your chemical anti-fuel. On top of it, you will have to maintain the compound, and prevent it from exploding from thermal shocks of the fire (adding another layer to the problem).
Also fyi, it is possible to spot and seam weld extremely thin metal sheets if the process conditions are appropriately controlled. Anyway, I suspect you will end up with a 22ga spot welded box, 'sealed' with ceramic adhesive or braze and lined with insulation. "Keeps your stuff at Xdegrees or below for a whole Y hours in a Zdegree fire. (Only tested for fires from A - Trash, wood, paper. C - Electrical equipment)"