How can I create low-cost, fire resistant, hidden in-wall storage?
Brick and mortar box.  Put drywall over brick if you want to hide it.  Leave an opening so you can take things in and out.  Can use an old fire oven door or something for the opening.  I actually found the best place to do this is in a closet ceiling if you have attic space above.  It doesn't have to look good so don't worry about the brick job - just make sure that is is pretty well sealed so heat/smoke can't get in.  Could use garden stones too.