This
thread has a link to an article about a British underground bunker system.
While not directly about the actual shape of Prime Base, it shows how a complex that was shaped to that of the local terrain was laid out.
Perhaps we are too wedded to a uniform or symmetrical shape for Prime Base? Has anyone seen any organic/conforming shape designs proposed like this one?
This
link has some good pictures of the same bunker.
This
(no longer updated) link has an interactive map that shows the extent of the space in Wiltshire.