Abstract: A composite underbody cushioning and ventilating structure of general use and especially useful as a seat pad for the occupants of vehicles, formed of two sheet members with padding material between them, one sheet member being of woven, fibrous decorative material designed for ready passage of air therethrough and the other sheet member being a formed plastic three-dimensional preferably air impervious sheet member that serves the multi-purposes of (a) spacing the sheet members apart for passage of ventilating air between them; (b) presenting an exterior surface that ventilates by lateral air movement and thermal current air movement within local areas of the member; (c) acting as a support for a body while contacting the body in measurably less than 50% of the area of the body; and (d) presenting an exterior surface having a decorative appearance. The present invention also involves the features of the formed plastic sheet member, per se.