


Merge all of the surfaces together ( Edit->Merge).Then I simply selected Insert->Boundary Blend and selected the green checkmark to complete the feature. In the image below, I started by selecting the edges along the open edge of the core (shown in red).

You're attempting to create surfaces to "close in" the volume you wish to measure.
NEGATIVE SOACE FILLER UPDATE
One method is better if you intend to keep this "core" and update it as your cavity updates. Neither is really easy to communicate in this format. Blank walls, corners, nooks and other spaces are right what you need for a harmonious look.I was able to generate the cavity using a couple of different techniques. It goes not only to minimalist spaces but to any other, too: from moody to mid-century ones. Negative space in living rooms is a must! Living rooms are usually large enough to leave some space blank, and you will make your room look bigger, more airy and fills the space with natural light. And the last but not least: fill in if the negative space doesn’t feel ok for you, then look for other spots to create one. Third, declutter your space – I think, this tip doesn’t require any comments. Second, look for functional and double duty furniture to leave some space unfilled. The first tip for pulling off this idea is to intend a negative space while decorating as when it’s intended, it brings harmony, and when not – it seems that you’ve just forgotten something. Third, a negative space will make your design and artworks stand out creating a cool balance. Second, a negative space is easier to use: you may place there whatever you want any time, and you can create various paths through your home, which is cool for open layouts. Plus, we tend to react to rooms with negative space more positively because they’re easier for our brains to categorize. What do we need such spaces for? First of all, negative spaces are psychologically pleasing: we all love balance and when there’s too much decor and furniture, we percept it like clutter. Negative space in your home means a spot where there isn’t anything: no art, no furniture, no stuff – nothing.
