Auralex MetroFusor 2 Inches Deep, 2 Feet by 2 Feet Thermoplastic Paintable White Acoustic Sound Diffusors, Box of 12
It's custom made from high quality EPS (expanded Polystyrene), so it's lightweight, affordable and can be easily painted using water-based paints ONLY (petroleum-based paints will 'eatthe expanded Polystyrene). They are also easy to mount and inexpensive to ship.