and these show insulating the back wall and covering with sheet metal…

When I made my block-off plate I got the metal cheep at RE-Store ( Habit for Humanity re-store), a air duct I could cut up. I used extra foil faced flue liner insulation on the the back of the plate. I over lapped the edges of the plate and foil taped the liner to the stove side of the plate.  The foil and insulation made a air tight seal. A little hi heat silicone away from the liner finisher the job.