Abstract: A container is utilized to hold fluid under pressure and has a non-circular arcuate opening. A cover member is movable from an open position through the opening to a closed position in which a seal on the cover member sealingly engages a wall of the container. The cover member has an arcuate non-circular configuration corresponding to the configuration of the container opening. When the cover member is moved to the closed position, it may be skewed relative to the container opening so that portions of a side wall of the cover member are spaced relatively close to a side wall of the container while other portions of the side wall of the cover member are spaced relatively far from the side wall of the container. An improved seal is utilized to prevent leakage of fluid under pressure from inside of the container around the skewed cover. This seal includes a body portion which circumscribes the cover member and is disposed between a rim of the container wall and a rim of the cover member.