In a container of the kind having an impermeable cover which is sealed to a base, air then being evacuated from the region between the cover and the base, an improved seal is provided for use with a rigid cover. The periphery of the cover fits into an endless groove or channel on the base, a temporary seal is then effected by inflating an endless tube or tubes in the groove and air is then evacuated from within the container so that the cover is drawn down onto an elastomeric strip in the base of the groove to effect a further seal which may hold even if pressure falls in the inflated tube or tubes.