plate 50c is provided with an inwardly projecting ring 50g that projects beyond the end of the pipe 50a whereby to ensure that the latter can never be occluded by another part of the bag when the pressure therein is below atmospheric as also mentioned above the bag is further provided with a vent 51 shown in fig1 as consisting of a similar arrangement of clamped plates but without a valve instead it has a simple cap 51a that threads onto the pipe 51b the drain 52 can be similar in construction to the vent 51 if found necessary the vent 51 or the drain 52 can be formed with a ring such as the ring 50g to ensure that the outlet can never be occluded by another part of the bag fig1 and 13 show the construction of a tray 70 which can be used in conjunction with the bag this tray 70 is formed from a rectangular blank 71 of similar material to that of the bag although it can be and usually will be of a much lighter gauge since it is not called upon to withstand any significant internal pressures or abrasive forces the material of the blank 71 is folded along the broken lines shown in fig1 to construct the tray of fig1 as follows as side and end walls 72 73 are folded.