it sure seems that a "wooden box" with the wall studs, ceiling and floor joist of 2x6 PT, on 12inch centers... sheathed interior and exterior (under floor included) with 5/8 or 3/4 plywood screwed to each stud...double layers of top plate and solid lintel over a steel door would be a viable option for a retro fit..you could skim it in sheet steel if so desired...if you could manage the height so you could get it under a garage door, and into a corner on a pallet jack you might have a cottage industry on your hand... and portable enough to move with you

a check with Texas A&M or Univ of Oklahoma, college of Civil Engineering should have the specs for such a construction