seemed silly trying to cram a square peg into a round hole, also the 1.5" rockwool cubes seemed way overkill for my little kratky plants, so I came up with these netcups to fit a 1/4 and 1/8th of a 1.5" rockwool cube which I found cuts up easily with a pocketknife.

Also included is a spacer to fit these where a 2" netcup would fit

since the rockwool is a nice cube, you can leave the netcup gaps nice and open, which should let the roots thrive! Also designed to print without supports, optimized for bridging!

I like natural PLA/PETG for anything in contact with the roots, since it's safe for food and no dies other filaments probably aren't a problem though, there's also FDA food safe filaments, but that's overkill IMO for indirect contact like this

for the spacer, any filament works, dark colors are preferred for light blocking

Fair Warning: the long version seemed to get a bit sloppy with how tall the walls were and might lead to failed prints, I recommend the stouter cube version which prints great.

Print Stats: 

Adaptor: ~12m, 2.42g of filament($0.04) 

Cube Netcup: ~20m, 1.97g of filament($0.03)

For the Wide-mouth Mason jar to 2" lid:
https://www.prusaprinters.org/prints/60322-wide-mouth-kratky-jar-lid