Each side has two aluminium plates but only one layer. So each side has two pieces that lay flat side by side to make up the total length of the board.
The batterybox snaps neatly into place between the 2 larger fastenings without any screwing. The top half of the box will be screwed on to close it. The screws will go through the other part and in through the aluminium sheeting to make it more sturdy.