My wife wanted a magnetic clean/dirty indicator for the dishwasher. We both like cats so I searched here on Thingiverse for cat silhouettes. I found the designs by @wslab and my wife chose Black Cat 10. I downloaded the .stl and was able to hack it in FreeCAD (basically traced it into a sketch) to 5mm thick extrusion, "CLEAN" and "DIRTY" text strings (using Arial Black) cut 2mm out of the front and back, and two voids inside for magnets. The magnets I used are 15mm x 6.5mm x 3mm, that I purchased from Amazon here (not an affiliate link): https://www.amazon.com/gp/product/B077BWCC8D/ref=ppx_yo_dt_b_asin_title_o07_s00?ie=UTF8&psc=1 Any magnet of the same dimensions should work. I added a chamfer the bottom edge to use to help grip when removing the magnet, but without fingernails it doesn't seem to work as intended.

I sliced it in Slic3r PE v 1.42.0 beta1 at 0.2mm layer thickness. Filament color changes after 2mm, and 3mm. The bottom filament was Hatchbox Red, the middle filament was Prusament Galaxy Black (my only black at the moment), and the top filament was Hatchbox Green. I encoded a pause to insert the magnets before the 4.2mm layer with the Slic3r Custom firmware. I've included the Slic3r project (3mf file) that I used. The current Slic3r PE 1.41.3 stable will load the project file including the custom layer start gcode for the magnet insert, but will loose the filament changes.

I printed this on my Original Prusa i3 MK3S with firmware 3.7.0-RC1. Pictures and videos taken during printing are on this Google Photo Album:
https://photos.app.goo.gl/fMW4xSKwLbmfoGNk6

2019-10-18 - Added STEP file to make remixing outside of FreeCAD easier, and updated the .3mf file for PrusaSlicer v2.1.0. Also added a version that has the words on either side offset from each other to allow better prints when using a transparent middle color.