Is there a way to create shapes with a solid outline (possible with a certain stroke)?

I repeatedly found myself wanting to highlight parts of a document by placing a frame (box) around it. It would be convenient to be able to define some vector outline that then is rendered with a certain width (stroke), regardless of geometric transformations.

Not sure whether it is clear what I mean: I of course could draw a rectangle and save it with an alpha mask, but that would not be a generic solution because each time I'd adapt its size to fit the content (e.g. scale it or change the aspect ratio), the stroke width would change.