Hi Daniel, I use 5mm acrylic, the size of the shadow is about the same. And the shadow goes away if I move the lamp behind the camera (like on your videos). It grows as I move the lamp from the camera to the top.
But, I realized this is not a shadow, but the rest of the page receives extra light from the other side! For now I use 1 camera setup, so I'll try to explain as "camera side" and "paper side":
If I put a black paper on the camera side glass and move it upwards, I can see that the paper side gets lighter where the paper does not cover, except the last 5-6 mm, where the "shadow" is constant. But if I let the paper go down to the bottom of the "V", the shadow line is "gone", because the whole opposite side is as dark as it was in the shadow area.
Meaning, the opposite sides provide extra light! I guess this happens with all scanners, isn't it? (Can be tested with a black paper.) I wonder if this extra light is useful or does more harm than use, as being generated or reflected by the opposite glass. Choices: cover with black paper and lose that extra lightness, don't cover and solve the last 5-6 mm problem. I should make a photo, so you can see how much loss of light is there.
I've this lamp, I'll post a photo from the current stage tonight: http://www.massive.eu/Hungary-faro-74902-21-31.html