An insert for use in the fabrication of a watercraft hull to form an inboard/outboard propulsion system passageway. The design of the insert incorporates a semi-rigid body having an inboard surface, an outboard surface, and sidewalls interconnecting the surfaces. The semi-rigid nature of the body facilitates the insert fitting tightly with the build up of laminate layers forming a watercraft hull while maintain dimensional stability during the manufacture of the hull. The insert is further configured to be easily removed from the completed watercraft hull to reveal the finished inboard/outboard propulsion system passageway.