Ah - I didn't think that the question was about the illuminated panels, I thought it was about the main supporting structure.
I took the photo of the illuminated panel that you referenced above - you are correct, it's a plastic sheet, milled out to accept 'grain of wheat' bulbs, with several layers of paint silk-screened on the front to display the illuminated nomenclature.
Today, it is becoming more common to use LEDs rather than grain of wheat incandescent bulbs.
Michael