Thanks for the detailed instructions, Bob. I'm not 100% clear on what you're suggesting, but I'm interested in how you do it. Is there a tutorial or something I can reference on YouTube?
Not everything requires a tutorial or a video. Attend: Most vehicle rear windows are a three dimensional object. Since you layout and produce signs in two dimensions you have to flatten the subject window into two dimensions so that you can see what shape to make your sign so that is will properly fit when it's applied. The simplest, and perhaps the only, way for most regular beings is to tape a length of paper over the actual object. I have a roll of 24" light paper that I use for this. On windows that are taller than the 24" paper I tape two pieces together. Once the window is covered with the paper take a sharpie and draw the outline of the window on the paper. Since virtually all windows have a distinct edge it's a simple task. When you remove the paper from the window and lay it flat you have an exact replica of the widow if it were placed in a large hydraulic press and flattened out.
Now get an image of your template into your computer. The easiest way to do this is to take its picture. Adding the framing square to the image of the template gives you a precise reference for scale.
Once in your computer, scale the entire image so that the image of the framing square is 16x24. The easiest way to do this is to draw a rectangle around the image of the framing square and map, power clip, mask, or whatever it might be called in your village, the entire image into this rectangle. Then make the result exactly 16x24. Now when you extract the original image form the mapping, masking, or whatever, it will be actual size.
Now you can shape your layout to fit this image. When applied to the tree dimensional object it will fit perfectly.
This is one of those things that's easier to do than to talk about.