Replacing the plastic casing is a reverse of the removal - the big flat top under the fixed plastic casing first (Image 1 red), then lifting up and pushing back until the lugs are in towards the bottom

You might find it easier to apply gentle outward pressure to the side of the printhead cartridge carrier tray (Image 1 orange depth markers)

I found it easier once I had the section of the cover (Image 2 yellow arrows) higher than the pin pad (Image 2 blue depth marker) to push it backwards

If you get the lugs in at the bottom and the top flat section of the cover is aligned and close to the fixed printhead (Image 3), now is the time to check the optical cable has not been disturbed. It should be vertical and the print head should not have any problems sliding side to side

Finally replace the screws (Image 3 orange) and you are finished with the cover