Godot Version 4.2.1.stable.mono
I tried using the addon with a portrait game, and scalates the image when traying to.
It happens to me with any portrait value. The error starts when the viewport height value is greater than the viewport width value.

My game is composed of a bunch of scenes within other scenes (as it should be done in Godot), and my camera is also a scene, so I can never click on it because the main node of the camera scene is not of camera type. It would be interesting for the addon to detect nodes that are scenes and search for cameras wherever you click (optionally) following the node tree inward.

In the Image 1 below I'd like to suggest to set the bottom left view for the camera up so when you dock the camera to the left side the information at the bottom left corner (visible in the Image 1 below) can be still viewed. The transforms info is particularly useful when rotating stuff around even with camera visible as well as moving objects around.

In the Image 2 you can see that there's a status box with information about the scene/current view which is also often useful which makes the right side not the best option if you want to view the camera, the transforms info and the box at the same time.
All this could be resolved by moving the left docking position several pixesl up.

Image 1:

Image 2:

I hope me description makes sense, otherwise it's a really cool plugin ;).

EDIT:
I suggest the margin's Y value to be set to 40 for both 2D and 3D.

I downloaded this addon and put it in my game, however it seems that the camera preview doesn't show any sprites or other objects that are children of a CanvasLayer (see image)

The idea was mentioned on Reddit about integrating with Phantom Camera. Seems like it'd be useful to have basic viewport previews of the PCam nodes, even if it didn't have the fancy guide overlay.

Currently the code expects either a Camera2D or Camera3D node, so that would probably need to change to something more generic like Node2D and Node3D.