I've tried selecting a rectangle and rotating the box with the rotate tool in a blank layer, but clicking in the box doesn't seem to allow me to resize the rotated box.
I am attempting to remove lens flare by subtracting a rotated gradient that is restricted to a box, but it is quite hard to get the size of the box correct before rotating it. Unfortunately, if I click in the selection box after rotating it, GIMP seems to forget that the box was there.
The advice given here: How to edit existing rectangle selection in GIMP? therefore doesn't seem to work. Is there any other way to resize a rotated selection box?