A cover device kit includes: a plurality of heat insulation components comprising a lightweight construction material such as synthetic resin foam capable of being transported in a disassembled state and then quickly assembled into a box-shaped heat barrier structure having six sides including an opening on an open side for covering an opening in a floor or wall of a building. Four of the plurality of heat insulation components 14, 16, 18, 20 each includes two complimentary connections for uniting with two corresponding adjacent heat insulation components of the plurality of heat insulation components when assembled into four side walls of the box-shaped heat barrier structure 10. The cover 12 serves as a fifth heat insulation component and unites with the four components 14, 16, 18, 20 that constitute side walls of the box-shaped structure and is situated opposite the opening on the open side.