The sheets to form boxes and trays of rectangular shape may be cut in different ways, according to where it is admissible to have a soldered seam. Thus the bottom may be made separately from the sides, having a little flange turned on the margin to be attached by a horizontal seam to the sides, which latter may consist of one long strip, bent to suit the corners and with only one vertical seam to join the 2 ends; or the bottom and sides may be-all in one piece, with triangular slips cut out at the corners to allow of the turning up, when there will be a vertical seam at each corner, and no horizontal seam.