(a) cover a substantial part of the opening,

(b) are mounted and arranged so they cannot be opened outward from the cage,

(c) have devices for positive latching in the closed position,

(d) are of solid materials, except for a viewing window,

(e) are arranged so that they may be closed at all times when persons are being transported in the cage,

(f) are mounted so as to provide only enough clearance at the door to permit free closing or opening, and

(g) are of adequate strength to withstand normal shock loads.