The Problem

It's not possible, to my knowledge, to make a picture-frame-like structure (like a vertical //walls)

A Solution

Add a //vwalls (vertical walls) command or similar.

Alternatives

You can use //set and //line commands but it's harder.

Anything Else?

No response

related: #2279
also see the comment: #2279 (comment)
notably these things are very poorly defined for non-cuboid selections

I had the same problem a few days ago when I had to make lots of nether portal frames. The best I could come up with was to make solid obsidian walls, then //contract 1 l,r,u,d and //replace obsidian air. Not useful in cases where you need to preserve what's inside the frame.

I'm in favor of having nice extra features for cuboid selections, because that's all I usually use.