I kept knocking over the test tubes from my API Master Test Kit when testing the water in my fish tanks so I made this to make them a little more stable.
It holds upto 8 tubes that are MAX 12.75mm OD
It says "fresh" on one side and "marine" on the other to prevent me from confusing the water samples when testing from both tanks of mine at the same time.
The fit is nice and snug, safely holding the tubes even when inverted, but not so snug that you think they'll break.
Prints easily vertically (left edge down to build plate).
I printed mine at 60mm/s 0.8walls and a very slow first layer, around 30mm/s and 20% infill.
Doesn't need support, although i did have two initial attempts come free of the build plate, so maybe you could try a brim support though in the end it wasn't 100% necessary.