Växer holder for starter plugs

This replaces the pod with pumice that holds the starter plug normally. I started using these since I was getting tired of replaceing the pumice stones everytime I restart my setup. Also it helps to reduce algae growths since there is less water surface exposed to the sun.

I made a [4 plug holder](https://www.thingiverse.com/thing:3336323) version to start seedlings in the tray.

Added a version with a smaller brim to be able to use the
Nursery Cross by fastriker.


For commercial use