IIRC when I was forming the patch for the bottom of my doors which had a similar bend to them the metal naturally wanted to form some curve to it as well. I was just using my bench vice and a short piece of angle iron with a MFH (a smaller BFH). I had to straighten them out but you may be able to go the other way with it. Maybe start with a longer than needed section and then cut out what you need?
Maybe a real metal guy like MPC will step in and tell us how its really done.
