two squares made of folded aluminium, both of different color. By folded aluminium I mean you take a fairly long plate of the metal (it can be something heavier than alu) which you would then fold into a square (or another shape) so that the ends of the plate are parallel but not touching (so yeah the object would be hollow) and then color it with something that shines like varnish paint (I would paint it in orange/green or red/blue , but its up to you). That is the Swiss school of design : simple, minimalist, effective. (oh and to make sure it can handle lots of books either glue a plastic base to prevent friction or chose a heavy material)