Line up the inside endplate panel to the shiny side on the outside end plate. Put a drop of weld on inside the slots and around the inside of the inside endplate.
Line up the inside rectangle to the inside edge of the end plate, to make a slot. Hold the rectangle in place and use weld on around the inside edge.
Place the longer piece of light diffuser film inside the track of one of the end plates, with the tabs facing away from the center cutout. Clip in the corner brackets to secure the film. Then clip in the flat springs so the film has a snug fit around the inside cutout.
Then place the shorter piece of light diffuser film around the rest