If you have a pet, you've probably experienced them begging for a meal when you're “positive” they have already eaten.  If you're not using your smart/cell/mobile phone to track such things, this could be a useful model for you.  Of course, it only works for pets that get two feedings per day.

The single magnet is 12mmx3mm, which I got from Amazon.

The check mark indicator is held in place by friction.  You may need to scale the check model up or down by 1% or so in your slicer to make it fit snugly.

If you have a single filament printer, you could insert a pause to change filament at a specific layer to achieve better legibility (see photos).

Since the two models have slightly different heights, if you are doing a manual color change, you should probably print the models one at a time.

I printed mine with white PLA and transparent red on an original Ender-3 about two years ago. No supports should be required, although if your printer doesn't handle bridging well, you might need to consider using supports for the magnet indent.