I started my second try on the front plate. Printing directly on the card board didn’t work out that well, so this time I printed it mirror inverted on a DIN A3 paper (the same size as the card board) and used spray glue to attach it to the card board. Then I used a crafting knife to cut out the characters, as before. This turns out to work pretty well, I guess the paper gives the card board even a little more stability.
Nevertheless this is a hell of work, the next time I’m doing something like this I have to come up with a better Idea for the front plate.