Hollow structure for breathing mask

A hollow structure, comprising:

• an inner wall;
• an outer wall spaced from the inner wall to define a hollow area between the inner wall and the outer wall;
• a bottom wall that couples the inner wall to the outer wall;
• and
• a first portion and a second portion which together define a closable opening into the hollow area, the second portion being coupled to the outer wall by a membrane,
• wherein the membrane is a bi-stable membrane that is movable between 1) a first position in which the second portion is spaced from the first portion to allow access to the hollow area through the opening, and 2) a second position in which the second portion abuts the first portion to close the opening,
• wherein the bi-stable membrane is configured to remain in the first position and the second position without application of additional forces external to the bi-stable membrane, and
• wherein the hollow structure is a cushion for a breathing mask.