the cap of the present invention forming the top layer fig1 17 illustrate another embodiment of the present invention having a cover 300 formed with a solid convex upper surface 305 and a generally smooth flat lower surface 310 the upper surface 305 may include a hot stamped cautionary indicia 315 imprinted thereon such as the words u201c caution u201d and u201c do not remove u201d the cover 300 is integrally formed with an annular stem 320 with an inner surface 325 and an outer surface 330 the outer surface 330 is preferably formed with longitudinally extending elongate ribs 335 spaced apart by regular angular intervals such as ninety degrees the ribs 335 extend from the cover 39 s flat lower surface 310 to a position beyond the midpoint of the stem 320 and more preferably about three quarters of the distance of the stem the ribs 335 prevent jamming of the device in close fit holes while providing a better fit for holes with higher tolerances the ribs also help to strengthen the stem and provide additional stiffness in a preferred embodiment the ribs 335 are trapezoidal in cross section to further increase the stiffness the ribs 335 can number anywhere from two to eight or more but a preferred embodiment includes four equally spaced apart ribs in a preferred embodiment the lower surface of the cover 340 that lies inside the stem 320 is arched to reduce materials but includes reinforcing members 345 to increase the loading capability of the device the reinforcing members 345 can extend across the walls of the stem 320 in a grid pattern to provide resistance to collapse of the arch forming the cover 300 in a preferred embodiment the hatch pattern is.