I originally made this for my wife to help her organize her sewing thread spools. It is probably not 100% universal, as I measured only spools she had at the time.

Base design is made of 9cm square blocks which can be connected together, each holding 9 large spools or 25 thin spools.

UPDATE 2019-01-06: Fixed connector size a little to fit more easilly between boards. Also changed default print orientation in STL, to be more rigid and do not break off easily.