Abstract: A carpet securement includes a polymeric body with a longitudally extending groove in its bottom surface. Tacks or other fasteners protrude upwardly through the body. The tackheads or other drive elements of the fasteners may be lodged within the groove and serve as a bearing element to help support that portion of the body bridging the groove. The body may also be provided with a downwardly projecting riblike support rail inside the groove to help support the body. A groove minimizes the weight of a polymer incorporated in the securement body and further permits flush seating of the fasteners without localized overstressing of the polymer. In an extrusion process useful in making the body or other polymeric elements, sizing dies are provided with devices for maintaining a fluid film between an extrudate passed through the die and the contact surfaces of the die itself.