You are right about sharing ideas on this forum so others can read them and apply them. That's how we learn.
Using four 12" fluorescent tubes on the second lightbox that you built surely improved the brightness and distribution of light. For what I need to do the light should be as bright and uniform as possible. The lightbox must be free of "hot spots" (the points where the bulbs themselves shine through the glass) That's the reason why a couple of inches between the light source and the glass must be maintained. I will be using a piece of acetate (opaque plastic) sandwiched between two pieces of clear glass so as to prevent the bulbs from being seen and glowing unevenly when the lights are on.