It does depend on the car. For a 30's sedan most substitute clear acetate and cut with sharp #11 to fit as it has a flat windshield.
You can also do this for curved windshields, it is a bit more work but Bill Geary showed us how a while back...he used epoxy and took his time with it.
If you want to cut clear plastic, wrap 1 side with wide masking tape (clean and dustfree surface of windshield) mark and cut slowly with a fine tooth razor saw.