These are small plastic parts for easy growing of Kratky hydroponics plants in plastic bottles.

I include three sizes: for narrow bottle throats (21.4mm inner diameter), wide bottle throats (33.4), and for big 6l bottle throats.
If you need custom sizes, I include a FreeCAD model file. It's largely stable if you increase the diameter, but can break if you decrease it, so it's set to 21.5mm at the moment.
I recommend adding 0.3mm to your bottle's diameter if its HDPE or other soft plastic, and add 0.2-0.25 if it's PET. You probably won't need to add anything for glass.

Print settings:

• I recommend PETG for easier inserting (and taking out; you won't need to take them out more than once per season though), but you can use good quality PLA too.

• No supports or brims needed.

• Prints upside down.

There are complimentary funnels, straight and curvy, that fit the refilling opening of these inserts, I'll link them here too https://www.printables.com/model/1722420-curved-and-straight-funnels-that-fit-my-kratky-bot

Bottles, inserts, a funnel, cotton wool, store-bought nutrient solution, and some kind of a propagation tray is all you need for this DIY system for balconies/outdoors. You'd need to get a grow light too if you are indoors.