Switch from 2 column back to 1 column inside boxed layout

In the new record layout, when working inside a box with multi-column configurations (e.g., A|B side-by-side), is there a clean way to revert back to a standard full-width 1-column layout underneath (like field C) without having to completely restructure or nest separate blocks?

Is breaking out of a multi-column row back to a single column inside a boxed layout currently possible?

[Box]
A | B
C
[Box]

If not, maybe having a 1col option in the settings would resolve this?