Save a contrasting image as PNG or JPEG. Convert it to SVG (there are many online applications to do this - [url=https://image.online-convert.com/convert-to-svg]one example here[/url]).

Open it on Tinkercad and make it as a negative shape. Yo can get this sample [url=https://www.tinkercad.com/things/fxOL5aMRypv]here[/url]. Export it as STL.

Print it out and it's done! You can even put magnet inside if you want. It might be useful!!!