I used a CNC machine to carve a triangular grid halfway through a sheet of 3/16″ foamcore. The result is a flexible sheet that I then rolled into a spiral shape. The vertical kerfs were glued to give the structure rigidity. The grid is not visible when the light is switched off, but is revealed when the light is turned on. The spiral shape just happened to create some very interesting (and unintentional!) light and shadow patterns on the walls and ceiling.