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.