I can't really flatten it without either excavating one end or raising the other end on a platform...
You need to flatten the terrain for the entire built up area. Usually you will stop at a wall or fence that's there in reality.
For reference take the level of the main entrance and flatten from there.
Just imagine that you are driving along the road and will enter the base.
In reality there are gentle slopes, yes, but you don't notice them and they will only cause us trouble.
Did that for dozens of building models for Google Earth back in the day and it works well.