At least the original poster, if he can't find a rust free donor, can rest assured there are other options for him, rather than a bondo/fiberglas repair that would be less than permanent. It's very likely someone skilled in metalshaping lives relatively close that could indeed perform this repair, but if you've already crossed that option off the list, then I guess no need in looking. I think your view of the fab method is rather pessimistic though. I've yet to experience the 0% assurance rate you speak of..
But if nobody ever tried, they'd hit that number every time
