These designs are tube holders for 1.5, 2 and 5 ml micro-centrifuge tubes. It is designed to hold individual tubes straight during weighting. I also use it to hold 5ml tubes on a magnetic stirrer.
The design objectives where to make it stable, light (for weighting) and to be able to see the inside of the tube.
For each size you can choose between the standard file or the "wide" one. "Normal" is design to be a tight fit where the holder does not fall out if you take the tube. The "wide" version is looser, the tube and the holder will not hold together.

Edit: I have added files to fit two more types of tubes (2 and 5ml see picture). The files are designed as "V2"