This is a fully customizable test tube holder.  The number, size, and length of the test tubes that this holder is equipped for are limitless.  Included are the FreeCAD files, and a base stl. file, equipped for four 10mm diameter test tubes.

To change the holder for your needs, simply open the FreeCAD file and begin exploring.  Things are labeled, and easy to change.  Most of the geometry of the holder are in sketches with very clearly marked dimensions.  There are some dimensions that need to be changed by editing the dimensions of a sketch extrusion.  These will be relatively apparent as you edit things.  When this is the case, you will be able to see the current dimension in the window below the part selector.

Average test tube holders retail for anywhere from 5$ to 25$, so this file, when printed can save you at the very least a few dollars, or upwards of twenty.