1 piece of polystyrene sheet black, size is around 300mmx200mmx3mm.The sheet will slide into the MakerBeam T-slot, MakerBeamXL T-slot or OpenBeam T-slot. Useful for more mounting options or if you want to encase your electronics. Polystyrene is easy to work with. You can drill or saw the material quite easily. It is also possible to cut it with a knife. At the same time the material is solid enough to give you more mounting options. The material can also be used to encase your electronics. One side of the sheet is glossy (covered with protective film) and the other side is mat.