How did I miss this one! Great build & paint job! Thanks for posting!
The extra curved piece was intended to go at the bottom to help create the curved surface. I orginally tried one and a lot of filler on the backside and them sanded the curve but the fill wasn't strong enough so I made the bottm section in two pies to make it stronger, but yours looks great as is.
For my own I added an extra strips of plastic to the inside-top so that I could round it off the top edge a bit and for the dent. Ended up at about a 1/4, this also really stiffened it up nicely.
Alan