Tried a test of concept.

I tried a candle but it wouldn’t light the whole word.  This is with a lamp but the light is still rather diffuse.  Nothing compared to the “Code Room”.

The thickness of material could be an issue.  The power of the light source could potentially be an issue as well.  Thinking of the dilemma brings one back to the days of physics classes.

Trying to imagine someone sneaking a spotlight into the CIA and getting the right position and there being a surface at the correct distance to allow a sharp image to be seen…

I’m having second thoughts.  Maybe he initially wanted it to be one big cylinder but changed it last minute when the CIA said “Are you freaking kidding?  You want a big light shining in our courtyard all night?”

I’m not sure sunlight would be enough.

Why not one monolith or obelisk instead?  Why go forward with the letter-cut panel?  If it won’t work, why still use it?

Maybe he just had a hard time letting go and thought it would look compelling enough to continue and then split it into the S-shape.  A flat shape would be convenient but that crazy bended S-shape certainly complicates any shadows.  Certain letters will be aligned or obscured…

Also possible that the cut-through allows viewing of the ciphertext/vigenere letters from both sides, however that might be significant.  It could be that simple.

I’m off to watch some V for Vendetta and mull things over.