Peter, as I have recently posted, a friend made one for me, using 3-D printing to make the housing he designed and the controller housing (which he is going to print if he can get around to it...). We had difficulty with light evenness and tried two sheets of diffusion but that made insufficient difference. Lining the inner housing between the panel and the diffuser with white paper apparently made none. The solution was to taper the light from the LEDs, making it brighter toward the edges. don;t ask me how; I'm not the programmer. It's not perfect at 4x5 full-frame but quite good and easily manageable. (I have certainly used enlargers with a lot worse distribution.)
I believe the panel is an inch from the diffuser (perspex/white acrylic), which I purchased from online in a 12"x12" piece and cut. It's 1/8" thick. I would have to open the head to check, which is takes a while. In any case, we simply laid the panel facing up on a desk, turned the lights out, and positioned the diffuser up and down, looking for the point at which the individual lamps disappeared.
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