Oh I see what you mean.

I've never been in the shelter so I cannot say exactly what the lower part look like, but I can say for sure that it is of a different shape than the entrance.

You can see two pictures of the shaft, in the "Construction" section of this post: https://londontopia.net/columns/lauras-london/lauras-london-exploring-clapham-south-subterranean-shelter-hidden-london-blitz-windrush/
One if from the building from this note, and the other, on Balham Hill, is from this: https://www.openstreetmap.org/way/432853031

The other pictures describe the inside of the shelter, but I don't know what the exact shape of the underground level is.