fit in the contour of the hollow space it is understood that the rigid support need not fill the entire cavity of the hook like lip and this is illustrated in fig6 in which the u shaped insert 28 is designed to fill only about half of the cavity formed by the marginal hook like lip 33 this would provide more 34 give 34 to the lip of panel 37 for easier insertion of an adjoining panel panel embodiments 20 30 and 37 all describe a confined aperture into which the rigid support member 25 can be inserted through a transverse end opening of the panel at the factory or construction site in fig3 the panel 22 exhibits a free end terminating to form a longitudinal slot for accepting rigid support member 25 which can be merely slipped under the lip 23 this hook like lip 23 unlike those earlier described provides for the facilitated introduction of the rigid support member 25 since the support member 25 can be merely pushed upwardly beneath the lip 23 without the need for telescoping it through the entire length of the hollow space the resiliency of the hook like lip 23 will cause a clamping action that will aid in the insertion of the rigid support member 25 into the described longitudinal aperture those of ordinary skill in the art will understand that the siding panels of this invention can be extruded or molded into a variety of shapes and sizes exhibiting various contours and aesthetic appearances the hook like lip portion can define a closed loop or an open loop.