Four short brackets at the selection's corners, clamped to the view, over a barely-there tint.
The same clamp as the frame option, but only eight line segments are ever drawn.
The quietest option, and the closest to the sketchbook feel. It marks the corners the way a crop guide would.
In case 4 the ticks sit in the view's own corners, which can read as interface chrome rather than as the object.