When a wide polygonal dural window is created, a short dural incision length is preferred by surgeons because suturing a wastefully long incision line during closure is troublesome. A locator to facilitate making the shortest dural incision when creating a polygonal dural window would be helpful. We geometrically analyzed the shortest incision design for a pentagonal dural window and produced a simple locator for intraoperatively implementing this design. The design for a pentagonal dural window with the shortest incision is the same as the design for a minimum Steiner tree (MST) problem with five vertices. The MST consists of three interconnected Steiner points (SPs) with three equal, radiating branches. We produced a template of the features of the MST for a polygon (MST template) as a locator. The MST template consists of several uniform Steiner units (SUs), each of which has an SP at the center and three wings that branch off of the SP, and each SU also has three slits through which the wings of another unit can pass. This mechanism allows us to freely adjust the distance between the SPs of separate SUs. In clinical practice, we can create the shortest incision design for a quadrilateral or pentagon by arranging MST templates combining two or three SUs. If we open a wide dural window, the total incision lengths created using our method are 1-5 cm shorter than conventional incisions. The MST template accurately and easily reveals the shortest incision design.