Wow great to see the LEDs coming along! For the diffuser you might try a plastic fresnel lens. You can rough up the smooth side with sandpaper to make it translucent or put a piece of vellum paper in there if the refraction of the lens is not sufficient and or the sanding doesn't make the thing translucent enough. If it doesn't work, at least you wont be out of a bunch of money. I use a diffuser similar to this behind the marquee of my mame cabinet and it works great.
You might also consider painting the exposed parts of the copper plate with white paint (or reflective paint or reflective tape). It wont hurt its ability to dissaspate the heat, but it will certainly help with the dark spots you are seeing. Doing this ought to help more than any diffuser, I would think.