Clarification on Structures vs. Buildings
"Goal: Engineer a portable building that, with appropriate in game tags, can have barriers, doors and augments."
This, for clarification, is a structure. Buildings and structures get confused often, so I'm trying to clarify this distinction. As you've seen, new buildings are probably not likely. In this case, all buildings are structures. Not all structures are buildings.
Structures are, by definition of Building Tomorrow, an area of a certain size (maximum 10'x10'x10', or multiples therein) that can be constructed at game. Structures may have augments attached to them with the appropriate item built and installed, as long as there is enough space to accommodate such an augment. If you want, say, a bar setup with a still, this can be done with simple structures (or, the within buildings that exist). Build a structure, add a door, have enough space, build out a still and put it in place. Viola, one bar!
What you will deal with as it pertains to structures are a few potential issues:
1) Doors can be attached, yay!
2) Doors can be broken through, boo?
3) Walls cannot ever be foolproof as the walls of buildings. Unless you're in a building, your wall is unlikely to be of building quality. This means things can get through gaps, cracks, etc if able. What they can't do is just bowl them over. If you have a fence at waist height, an NPC can step over it. If you have walls with any permeable area, NPCs can get through them if they fit.
4) Ceilings - if something is coming through a ceiling, you're likely screwed, because things that come through ceilings usually do so at high velocity. But if it's a solid structure that has no way to get in, and the walls are built in such a way that there are no gaps and the like, you may keep smaller things out.
5) Floors - again, this is another WYSIWYG feature. Have a tent on the ground? Things can go right through canvas. No floor? No barrier to entry for digging things. Solid wood, metal or concrete floor? You may be safe. Maybe.
Here's the thing folks - the game is meant to be played within the system. The system specifically doesn't make "structures" super awesome because a) the system is not meant for "build giant barriers and defend yourself from within, spending no resources to stay alive", and b) the game should be played in such a way as you're looking for ways to survive, not ways to "beat the system". The creators of the game have specifically tailored the systems we play with so we can play to have fun, and ultimately play to lose (all characters die, it's just a matter of time).
That said, I highly suggest doing what's fun, and taking what's in the rules at face value. Play to lose. Do the dumb. Enjoy the Gravemind while you can - it's the only time an ST will sit with you, create specifically geared and tailored plot for you, and give you a chance to shine.
Hopefully that should answer the questions on buildings versus structures, and how they work on site.