My 8x8 matrix was larger than the one in the original project so I took the opportunity to redesign the casing. It was also in a serpentine LEDs arrangement rather than the L-to-R, row-by-row configuration, so updated matrix code file available at https://github.com/GSVNoFixedabode/CodeBits/blob/main/HourStrings_EN.h

The Instructables project is at https://www.instructables.com/VERBIS-Desktop-8x8-RGB-LED-Matrix-Word-Clock/

The stencil needs to have the first 2 layers printed in white, before changing the filament to a dark colour. 0.2mm layers give the best result, although the outer and lid can be printed at 0.3 if you're in a hurry.