The “assortment of openings” that sit on the base, is that cabinet more shallow than the base? Does it need drawers as well as doors? As it stands now you can draw an upper cabinet on the base and specify no doors and no holes of any kind. Then use the Insert a Door and Insert a Drawer tools. But you would need to add mid rails, mid_stretchers and fixed shelves manually.
You question has me thinking. If we had a divided upper cabinet people could design cabinets that looked like old style Hoosiers.
I am going to look at adding that capability. Thanks for the suggestion.