Abstract: A box is formed of a blank of sheet material wherein a filler is sandwiched between an inner liner and an outer liner. A pair of V-grooves is formed parallel to each edge and another V-groove is formed parallel to the pair of grooves on the side away from the edge. The V-grooves of the pair of V-grooves have center lines spaced apart a distance of substantially two thicknesses of the sheet material. The blank is folded upwards at the single V-groove, and then is folded back at the pair of V-grooves to produce successive 90-degree folds. The outer liner extends continuously over the outer side, top edge, and inner side of the resulting double-thickness wall. A similar technique can produce a box with multiple-thickness walls.