Flat keycap is intended to be customizable by the user! A project file is loaded as an example, as well as the keycap alone. 

1. Load the keycap model.
2. Load another design or model as a negative volume. (for best results, keep design around 10mm in size and 2-3 times layer height)
3. Slice as normal and enjoy your customized end cap!

Alternately, for a more tactile feel, you could print a small design separately or in an alternate color and adhere it to the top of the key afterward. 

Drawn by me in fusion 360 based on original CAD drawings from Cherry MX switches.