There's a handy tool for replicating curves that use a series of plastic or metal fingers in a sliding mount. One presses all the fingers to one side, then presses the bracket to the desired surface until all fingers are in contact. The opposite side now represents the measured curve and can be traced into a modeling program.
For your purposes, that is unlikely to work, as the tool is perhaps six inches long, far too short to span your windscreen.
Consider the following:
Attach a string to each edge of the windscreen or wedge a thin rod from one side to the other. This provides you with a straight line reference. Measure as nearly as possible to perpendicular from the reference string to the windscreen surface. I have a laser measuring device that has 1/16" or 1 mm accuracy above certain distance, which would not work well in this case as you approach zero at the edges.
Start in the center, record your measurement, and move an appropriate distance outward. I suggest you need only approximately 20-30 mm spacing as you are collecting points on a curve, which are likely to be relatively uniform and easily reproduced in software.
Your graph now has zero, zero on the left, and say 750 mm, 40 mm on the right, with matching numbers in the middle. On real paper, turn it into a graph, or use a suitable vector editing software such as Inkscape and duplicate it there.
Depending on the shape of your window, you may have to replicate the process at different heights in order to get a proper representation of the curve of the glass.
Once you have the dots, run a Bezier curve through the dots. The resulting like can be extruded or otherwise converted into the segments necessary for your 3D printing project.