The problem is that the donor car is likely to have rust in the same area! I have fabricated this piece a couple of times by doing it in stages--a separate piece of metal for each surface, then the whole thing welded together. If you don't have a MIG or TIG welder you may be better off paying a body shop to do that bit.

I fabricated a piece that fit relatively well, but it took a lot of time to get it right, and will require some time and filler to make really smooth. If you can get them from a donor car, by all means do so.

When you cut out the old piece(s) from the receiving car, be mindful of the drip rails, the bonnet if that is still in place, and possibly the rail that holds the under-bonnet rubber seal if you have to go that low.