A rotatable joint, comprising: at least three bar connectors; an axis piece comprising a central pivot and at least one of the at least three bar connectors longitudinally aligned with the central pivot; and two rotatable joint pieces each comprising a bushing and at least one of the at least three bar connectors oriented along the diameter of the bushing and aligned along the axis of symmetry of the bushing, wherein the bushing of each of the two rotatable joints pieces is configured to slide over the central pivot thereby coupling the two bushings with the central pivot and allowing the rotatable joint pieces to rotate about the central pivot, and orient the bar connectors of the rotatable joint pieces perpendicular to the bar connector of the axis piece.