Is this an AD series cab or 39-46 series? I have never seen factory metal interior cab metal corners for below windows on an AD but the earlier cab design has them and repops are available
For the AD cab, I made my own.
https://67-72chevytrucks.com/vboard/...d.php?t=841095 post 2
It could be a lot simpler than that in design and if you use light material like 20 gauge just bend it around a welding tank or something. Leave extra material on the door post side where the radius is tighter for leverage, take your time, lots of small bends, trim to fit once you have the radius right.