Im very late coming to this thread but i have to add that I’ve made a few light boxes (too many artists and photographers in the family.)
The secret to diffusion of light is to avoid any contrast inside the box. If you paint it white inside and smooth off any edges (bathroom silicon works well) then more light hits the translucent screen at an angle and illuminates more layers of the diffusion plastic. Placing the light source out of line of sight also avoids bright spots.
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