This was designed to hold test tubes that are 3 cm  wide and about 10 cm tall.  Because there is a recess in the bottom for each test tube, it will work with either flat-bottom or rounded-bottom tubes.  It is meant to be printed in two parts so that no supports are needed.  The legs on the top piece are hollow so that the legs of the bottom piece can slip right in.  I filled the hollow legs with  Gorilla Glue's "Clear Grip" adhesive before inserting the solid legs.  The test tubes are being used with high school science classes and are holding up very well.