I don't need to spin test tubes, so I redesigned the test tube attachment to be a simple open box-type container, specifically to be matched up with my separate shaker container. See photos of parts and assembly.
Kudos to the original designer of the hand-cranked centrifuge Serge Nader for the original design.
I'd recommend printing it at 100% infil out of PETG/ABS/PC.
The author remixed this model.