I came up with this a few years ago using Bryce.  Sometimes after a long render, you might find an object floating a half inch above a table, or you really need to add, move, or correct an item. Instead of re-rendering the entire scene again, I place this shield right in front of my camera, blocking out everything except the portion that needed correcting. Looking through the opening, I can stretch and scale the shield until it fits my needs. Just used it for the first time in Carrara to add some additional objects. Cut a five hour re-render down to 30mins.

The shield should be solid black with all material settings at zero. Rendering again at the same size, the black area will go a lot faster than the full scene. Afterwards, in your image editor, copy and past the corrected render over the original and delete the black area.

In the zip is an .obj file, (plus a .png thumbnail image and a .daz file for D/S users.) 9kb in size

Spot Render Shield

I hope this makes sense. The thing works for me and has saved hours of render time.