The reality bubble moves. So if you managed to make a perfectly reality bubble sized wall, with a “designated clear” inside and then proceeded to stand perfectly in the middle, and not move in any direction for days on end… then what? Soon as you move 1 tile in any direction, all zombies along that wall can attack it if they want. When you get closer more zombies can attack the wall, when you move to one side or another to take care of zombies further along the wall zombies along the other sides can start attacking etc…
Of coarse all this is assuming that whatever system that gets worked out allows for a single area that big to be made. And it also does not take into account you leaving the “secured area” for whatever reason, say, looting. Leaving would mean that mobs are free to approach the secure area/attack walls/ walk in any openings as soon as they are in the reality bubble again, they just can’t spawn inside.
might have to take special consideration with hordes, to make sure they can’t “spawn” inside, but can still walk in once inside reality bubble.
The allowed size for any “secured area” also has not been taken into consideration. I don’t see why an area would be allowed to be any bigger than reality bubble sized, probably not even as big, and would probably be best if it forced open doors to extend the “secured area” or break upon original creation of “secured area” (not of the doors) This way a secured area could be multiple rooms. It would require all exterior doors to be closed, and at least one interior door to every room be open though. I am thinking some sort of easily craft-able flag, or other claim device, or object that can be (e)xamined to change/ delete the secured area or something.