Craig Tube adapter for any straight-sided tube, including Eppendorf tubes.

thingiverse

An adapter for any straight-sided test tube to transform it into a makeshift Craig Tube for extracting crystals. If you have crystals in a tube mixed with liquid, simply attach one of these adapters on top, insert a cocktail stick into the bottom, and centrifuge it with the stick positioned at the bottom. After a short time, you will be left with nearly dry crystals in the tube and the liquid at the bottom of the centrifuge container. It is essential to choose a filament material that is compatible with the solvent used. To use this adapter, you will need to re-scale the .STL file in your slicer to ensure it fits perfectly on your tubes. The adapter should not leak when held upside down, but it should release a few drops of liquid if you vigorously swing it in your hand (testing with water is recommended). My initial attempt can be seen at: https://www.sciencemadness.org/whisper/viewthread.php?tid=83542#pid522683