Hi John,
To answer your question, common wisdom including those of Schroeder himself have the channels aligned with the sound, that is to say front to rear for the sound to travel through. This puts the broad horizontal diffusion side to side and the tight vertical parallel with the front and back walls. When I have painted them I have used "Kilz All" primer for the first coat, then any latex or enamel for the finish. The back of John's is finished with heavy duty black enamel usually used for wrought iron fences. It is very tough, but takes two days to totally dry! Rob in West Virginia has a large diffuser that is painted with latex. Check out his gallery to see the results. The material is solid high density polystyrene. It is cut from solid blocks with an accuracy of .001" over the whole surface. I think it is impossible to achieve that accuracy with glued together pieces. The dimensions are 2 ft wide by 23.5" by 6.5" total I have tested it in both 2 foot grid and 2X4 foot grid.
