print at 0.12 layers so there are more layers stacked. the tutorials for designing light boxes recommend 0.6 for the colors that will pass light and black being thicker to ensure good light blocking.
Edit - the Contour setting helps but found a youtube video where they said to turn off elephant foot compensation. Contour definitely strengthened the print but it also seemed to make the letter edges “blurry”. It looked ok, but just not crisp. But Contour definitely fixed up a butterfly light box. Reprinting without elephant foot compensation (it was on for this print unless I accidentally changed it) and no edge fuzzy/contour.
It adds a lot of time to a print but slice with it on and off and look close at the filament paths and you’ll see what I’m talking about. Point distance 0.4 and skin thickness 0.1 were also copied from the settings used in good prints.
A good design and a different approach to light boxes with the back incorporating the walls which made it easier to add lighting. It’s a fairly thin design too but perfect for a picture frame stand. Still thick enough to get even illumination. The colors have fewer layers than the black in the design but it works.