That turned out great!! Kudos!
As far as the bottom edge is concerned... Doesn't look like there's to much curve there. Not sure if your plan to weld in a piece of rounded sheet would work though. You have to remember that not only is the shape of the rear rolling under there, but it is also not a flat surface from the drivers side to the passengers side (does this make sense? Early on Saturday morning with only 1/4 cup of coffee and having trouble finding the correct words to desribe what i'm thinking).
I think if you were to try and weld in a pre curved piece that you would have a problem wth it buckling at the center of the car. I think you'd be better of tacking in a flat piece along the top edge and bending it as you go to get the shape right. This way you can prevent that "buckling".
The only way i see your plan working would be if you were to try to weld in two different pieces (left and right), which would be dificult to make it look right where they would meet in the middle.
Any of this making sense?
Imagine taking that curved piece of metal your talking about using, holding it in both hands horizinatlly and bending it. You can see how it would buckle in the middle.
Jeesh, i'm rambling... off to drink more coffee! good luck! Looks GREAT. No doubt others will be doing this!