UMG constraints - inherit width/height from other widget

What I’m looking for is a way to link two dynamically sized elements.

Img #1: An example case: the vertical box should align with the leaf element of the horizontal box.

Img #2: Nesting via canvas panels/scale boxes don’t work in this case

The only way I can think of how to do this is by setting the width of the vertical box directly in c++ during construction. Please let me know if there’s a better way!