I made this about four years ago and figured I'd share it. I always render to disk and sometimes find a spot that has a mistake, or some problem that requires re-rendering. With this shield, I can re-render only the area in the open window, without having to take the time waiting for an entire image to render. Especially handy if you have glass, reflections and other time consuming areas. It's like the plop-render, but you can save to higher resolution, unlike a screen render. When I made my mirror to mirror infinity image, I made one large render at regular settings and then with the shield, rendered the center of the reflections at a higher setting. Saved a bunch of time.

Just put the shield up close to the camera, highlight the cube, then resize and move around, as needed. Aimed the mini-tute for the most basic user.