Sketchup model. Imported it into the free 'DesignSpark' solid modeller and just projected the side of the box on the piece of MDF I had available. From this I created the following planview of the side:
After I created the two sides I just estimated the optimum width by just holding the monitor next to one of the plates and finding good position. 50 cm seems like a good width so I just made the second MDF board exactly 50 cm wide. By doing this very carefully and making sure all corners of the board are now exactly 90 degrees it will be easy to create all front, top and rear surface plates.
And then we paint..and paint
In the mean time I keep refining the 3D model. And it's actually progressing faster than the real thing: