Rigid or semi-rigid containers of polygonal cross-section, e.g. boxes, cartons or trays, formed by folding or erecting one or more blanks made of paper by folding or erecting a single blank to form a tubular body with or without subsequent folding operations, or the addition of separate elements, to close the ends of the body with end-closing or contents-supporting elements formed by folding inwardly a wall extending from, and continuously around, an end of the tubular body