Thanks to w3rN42 for the (remixed) model / hydrophonic screw cap for netcup's.
https://www.printables.com/de/@w3rN42
As well as the link to the model:
https://www.printables.com/de/model/423128-modified-planter-netcup-for-hydroponic
Unfortunately, his model requires buying suitable netcups (e.g. from Amazon), which I didn't plan on doing when we already have 3d printers :)
So I quickly created suitable / compatible cups.
These snap into the screw cap and sit tight enough.
There is also a slot at the top edge for easier removal and assembly.
Then turn the cup a little to prevent it from falling out.
Have fun with the part, greetings from Switzerland.
The author marked this model as their own original creation.