Emmanuel, thank you for your prompt reply!
Ok, I tried with d = 0.32 on a 20x20cm empty box (with 1mm thick walls) that I put in upside-down position on the floor of a dark room.

Inside the box I put a 10x10cm light source hanging at 10cm elevation.

Unfortuntely the resulting Q4 photo is still far from my targert: there are black shimmers on the box surfaces and some light escapes from the bottom of the box [:(]

I would have sent you an image, but sorry, I just cannot find the proper "attach file" or "upload image" option....