This is a test tube rack remixed from [https://www.thingiverse.com/thing:4461677](https://www.thingiverse.com/thing:4461677) .
My daughter got the "Chemielabor C 1000" from Kosmos and we wanted to have a proper rack for the test tubes with 20.6 mm diameter. We additionally bought some cheap test tubes with 16.2 mm diameter. I used the original model for the design idea and dimensions, but drew the rack from scratch in FreeCad . Make sure to print it on its side.

I had to sand the hole a bit to make the tubes fit. You can also just adapt the attached FreeCad file to your needs.