I mentioned this recently in another thread, but no one came through to confirm/deny or expand on it: There were reports some years ago that a section of a Z front fender has a contour that replicates the under-battery section of the inner fender.
Unfortunately, I don't recall that suggestion being accompanied by pictures to show how it was done (or whether it was done). Even if true, Z front fenders don't grow on trees these days, so it would probably amount to a difficult or expensive solution (or both). However...
The general concept is interesting. Perhaps a re-pop or scrap front fender from a more pedestrian donor vehicle would serve equally well? It would need to be a vehicle (car or pickup truck) with a similar, rounded styling. However, I think you'd only be able to size up the feasibility of this approach by cutting out the Z inner fender repair area and then eye-balling the shape, hoping that it would ring a bell: 'You know, that looks a lot like the crown of the front fender from a 1986 Ford Taurus'.
A DIY alternative would be to take a fibreglass cast from the repair area and then do some tree-stump/sandbag tin bashing, using the cast as a shape reference.