I made the diffusers by securing a long wood screw into the centre of the panel and moulded plaster around it to the approximate profile. Then using a profile template as a guide, sanded it into shape. The board was painted with PVA and the plaster applied while it was still wet. PVA was also mixed in with the plaster. The shaped plaster was impregnated with wood hardener to seal it and then sprayed white with matt acrylic paint.