Abstract: To provide a smoothly conically converging edge portion of a former, with air communication openings therethrough, the former sheet is bent over at the edge in the conically converging shape with the inner bent-over portion left free; core elements of conical shape matching the cone shape of the bent-over portion are introduced into the open space, and a curable resin such as epoxy, polyurethane or silicone is poured over the core, to adhere to the bent-over portion and the major wall portion of the former and, after dissolving of the core element, leaving air chambers to which air with differential atmospheric pressure can be introduced through connecting stubs which can be cast into the closing-off wall. Solvent, such as tetrachlorethylene to dissolve a polystyrene core can likewise be introduced through these stubs. If a plurality of core units are used, a plurality of separate air communication chambers can be formed with separating walls cast in the gaps or spaces between core elements.