Closures for pressure vessels, and associated systems and methods

A pressure vessel, comprising:

• a first flange having a lower surface and a first inner surface;
• a second flange having an upper surface and a second inner surface;
• a sealing member including:
• a first portion extending in a first direction and configured to contact the lower surface and the upper surface, the first portion including:
• a width, and
• a first thickness,
• a second portion extending from the first portion in the first direction, the second portion including a second thickness that is less than the first thickness, and
• a third portion extending from the second portion in a second direction that is transverse to the first direction and configured to contact the first inner surface and the second inner surface, the third portion including a third thickness that is greater than the first thickness;
• and
• a fluid port extending through the second portion in the second direction,
• wherein the first portion provides a first seal between the first flange and the second flange, and
• the third portion provides a second seal between the first flange and the second flange.