Braille probably takes a while to learn, and my older friend isn’t about to try. So when he asked for some CDs of cheery Broadway musicals (yep, some of us still play CDs), I glued “indicator letters” on the jewel cases so he can tell them apart, e.g., “G” for Godspell or “MF” for My Fair Lady. I can’t think of an alternate deployment of felt letters, but you might.
Two glues worked: Mod Podge and super glue. A glue gun might have melted the felt or plastic. Three letters of readable size took up too much space. Cardboard wasn’t thick enough to read. The thicker the felt, the more readable. If the colors are ugly or your stenciling sloppy, well: they’re blind. Don’t forget a nice cup of coffee as you craft.