Round-to-rectangular duct

A hollow duct adapter that transitions from a round opening to a rectangular opening, designed for FDM 3D printing in PLA. The round end (default 100 mm diameter) connects to the rectangular end (default 1" × 5"), with straight collars at both ends for slipping onto hoses and a tapered loft section between them for smooth airflow. All dimensions refer to inside (airflow) surfaces; wall thickness is added outward. Adjustable parameters include both opening sizes, corner radius on the rectangle, wall thickness, collar lengths, and transition length.