Abstract: A fiberboard panel is formed from at least one corrugated sheet of molded cellulose fiber material having alternating ribs and grooves. Spaced slits are cut through the ribs on one face of the sheet and terminate short of the opposite face so as to form at least one slotted portion of more flexibility which may be bent to form a curve. The corrugated sheet is sandwiched between two flat sheets of the same material to form a composite panel. The corrugated sheet or panel is formed by a wet processing method including successive cold and hot press steps in which a wet mat is pressed between an opposing perforated platen and non-perforated, elastomeric material platen of cross-sectional shape substantially matching the desired panel shape and dimensions.