Step 2: The size of the wood pieces
The length of the side parts is the 500mm minus twice the thickness of the birch multiplex. So if you only get Multiplex with a thickness of 14, adjust the length of the side parts to 472mm (500mm - (2*14mm) = 472 mm)
The size of the 4 stabilizing sticks is not so critical, they can also be 280mm long and 10mm or 14mm thick, you'll see later why. Buy a cheap rectangular block of wood and cut the pieces by yourself, thats quite easy. Multiplex is very stable in large pieces, but if you cut it in small pieces, the layers might disintegrate!
That's all, let's start!