This is a square solid inside a triangular hole within a triangular solid inside a square hole.The predecessor to this design featured a triangle inside a square hole within another triangle inside a square hole. It sounded nice but the square hole broke up the symmetry of the triangle, plus it didn't exploit the reciprocal capability of a reuleaux triangle to contain a square inside a triangular hole.As with its predecessors, the slots have a 45 degree overhang that should suffice to be buildable on FDM while still locking the parts together.