After some unsuccessful experimenting with materials like Polystyrene (cracks too easily), I settled on 0.3mm white Polypropylene which is both easy to work with and easy to find. The light becomes a little bit blue-greyish cold. I bought Ibico PolyOpaque report covers for this purpose, which you can get at German Staples stores. You can get it in 25, 50 or 100 DIN-A4 packs. Because only two full-size pieces can be cut from a single A4 sheet and you need 30 pieces you need at least 15 sheets for a single full-size lamp. I built 10 lamps in various sizes from this material and it seems to work pretty well.