A major problem with original canon Prime Base was the cover story. No one likes the published one and no one has come up with one that hasn't been shot to pieces. Let's go back to first principals and see what we need and if I have a solution that works.
The base needs several types of entrances. One needs to be the construction entrance, so that is what we will chat about here.
This needs to be
Concealed
Secure
Big enough to allow a lot of large machinery and a high volume of materials to pass into
Not arouse suspicion as the activities take place
Either be concealed or filled in after it is done serving its purpose and before the war.
How about this
Grain elevators are all over the USA. They are huge. They have internal railway spurs that allow cars to be loaded and unloaded in covered conditions.
Morrow Industries owns the grain elevator. Some of the cars that come in are carrying supplies, equipment or personnel for Prime Base. They are taken into the buildings and detached from the train and put on a small siding that is in a building and is on a large elevator. The car descends to a deep tunnel and is pulled to the base, which could be several miles away. The car is either returned to the surface or cannibalized for materials for the base.
Any issues?