I'd probably be living under a bridge somewhere, as slow as I am.

I hear you on that one. But... slow is the way to go with sheet metal, so carry on.
As far as matching the existing curve, I would say to try a flexible shape pattern to make a profile, but I do not know how to explain it very well, or if it would be appropriate in this case.
Also, that stump looks like a purty one to me. Keep posting your progress; it's cool to read along.
Brad