Go to the setting menu (gear icon) and click edit walls.
First step is removing the walls, pointer will show closest wall, click trigger to remove, you must go left to right.
You are prompted to hit a or b when finished, and it will switch to wall creation. Then you need to set your new walls and make sure they eventually connect back to the final position (indicated with red marker) game will then do a save, and return to the normal settings menu.
Now you can add cubes from the same menu, go into the space and mark out the rest of the stuff.
Your thought about doors is wild. Thanks . Been thinking a lot about door frames as potential special areas/used for anchors/identified in a different way.