Is there a 3D Alef-Bet? In which case Samekh could be something like a sphere, etc.
Or for that matter something 4D like kinetic sculptures, which would explain how the aseret ha-dibrot were legible from every direction . . .
Could this be related to the form of the actual sound waves in the air when the letters are pronounced? In which case people would literally be seeing the physical forms of the sounds, explaining the synesthesia as well?