3 onto container 1 to keep cover 3 in place with respect to container 1 and can also shield cover 3 and container 2 where they meet reference will now be made to fig2 wherein a portion of cover 3 and faster 5 is shown with gas outlet 7 and a retainer 10 are shown retainer 10 preferably prevents fastener 5 from being shifted out of place and preferably from being inadvertently released from container 1 in the example shown in fig2 fastener 5 is placed on container 1 so that a notch generally indicated at 12 of fastener 5 receives a pin 13 of container 1 once pin 13 is received in notch 12 fastener 5 which can either be connected to or be independent from cover 3 is preferably rotated thus permitting pin 13 to enter a channel generally indicated at 12 a of notch 12 therefore once fastener 5 is in this locked position neither fastener 5 nor cover 3 can be removed simply by pulling at it rather fastener 5 must be rotated until pin 13 is no longer within channel 12 a of notch 12 before fastener 5 and cover 3 can be pulled away from container 1 however an additional verification that cover 3 is properly in place is provided by retainer 10 in accordance with an embodiment of the invention in the embodiment shown in fig2 and 3 b retainer 10 comprises a projection 11 which can be inserted into a corresponding aperture in fastener 5 preferably container 1 has a corresponding aperture or cavity for receiving projection 11 therefore when projection 11 is received by fastener.