If you want something that works like a director's finder, the Linhof finder is it. I use it exactly as you describe (as well as for its intended purpose--as a finder for press-style shooting with a Technika and rangefinder). There's also a similar device made by Horseman, and there are some finders made by Fotoman and Gaoersi.
The older style Linhof finders aren't too expensive. The older one crops the image as focal length increases. With the newer style, the frame stays about the same size and the image zooms in and out as you adjust it. I've owned both, and I think the new one is a worthwhile improvement, if you use it often. Both allow you to adjust for focal length and subject distance, which I don't think the Fotoman and Gaoersi finders do. I have an older Gaoersi finder for 6x17, and it has focal length adjustment, but not subject distance, and the distortion at the ends of the frame is pretty bad. I gather the Fotoman finders are an improvement over the Gaoersi finders, but it could also be that newer versions of the Gaoersi finders are better.