and yet more...
After thinking about how to do the letter mask for several days I hit upon the idea of just inverse & backward laser printing them on a piece of white paper and sandwiching this between a piece of 3mm thick dark tinted acrylic and a 2mm thick clear acrylic. The dark tint works well to hide any detail when viewed from the front of the clock so you don't see the letters. The 2mm clear backing sits directly on the LED's as I had calculated the LED spread so it lit the entire letter and not just the middle of it.
