1. It will not fall into the hole, even if rotated, tipped, etc.
  2. It has no corners to bend, break off, etc.
  3. It does not have to be carried, but can be rolled.
  4. Human beings are roughly round in horizontal cross section.
  5. Orientation of the cover with the access hole is not of concern.
  6. Orientation of the access hole with the ladder in the pipe below is not of concern.

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