Depending on how much needs to be repaired,there was an option I used
Obviously I am working body off, but think this could be done body on. I used single panel 18ga, folded over top edge 90deg ( there is an inside body brace/cross plate on the inside of the cowl, the top of my replacement plate fits up against that cross brace ). The bottom was cut longer length to allow for needed bottom plate bend up. Excess length rolled up to form the bottom plate curve, ( was able to template the curve arc and scribe/ cut panel.) I placed the panel up the inside, leaving all the original metal in place ( I am retaining the " vehicle patina as much as possible ). Confirmed the curve radius, pulled out and welded the plate curve.
There was sufficient original metal inside along bottom in place along the bottom curve to provide solid weld connection- that was important, had to be there for weld connect, but it's multi plated original construction there, so odds are it's there. I also spread panel adhesive between the old/new panel, slid it up inside and then and put a spreader bar to put pressure on plates. Was gonna drill the to plate and bolt it to the inside cross brace, but was able to use self tapping screws as I recall. Will also have a bolted on gas can carrier on that panel, so more securement of the new inset panel. I was also able to tap weld along the bottom rusted original panel, grind, light body filler, and then some rusted red and green spray paint to " patina it.
Original outer panel wall with slight weld /body filler along bottom. The two bolts on side were to a tool box originally mounted, will replace it or put gas can mount there
The passenger side done the same, but also put in the inside panel on just to strengthen it - had slightly more rust deterioration.
You can see that I also left the " rough metal" finish. But, thinking you could cut out any bad metal and tack in the new formed cowl, just as would be required if you purchased the formed cowl. Either way, made or purchased, you gotta cut to fit, ( also, need to place the holes and nuts for fender runner ). Hope this helps.
Oops, Looks like this should be moved to garage