Summary

This is a hydroponics remix of O3D's self-watering planter.

It fits this sort of 7cm white-label water level indicators with an outer diameter of 14mm:
https://www.dehner.de/produkte/wasserstandsanzeiger-M200006794/
You should be able to find similar ones at every other garden market.

Don't save on infill for the reservoir, otherwise it might leak.

Have fun!