[freebeard]

The bracing is a good idea, and the tabs in the trunklid gap is a neat trick.

I expected the side pieces to be your big challenge, but the result looks like you used a heat gun. Right? The cost for aluminum sheet would be similar to new plastic. Try cold rolling [either] before you attach the pieces, or even trim the final shape. The tool would be two rollers on the bottom, with a third that you can press down from the top incrementally as you push and pull the sheet through. The roll would start on a line from the top front to lower rear corners.

Given the size of the pieces you might want to just take that step to a sheet metal shop.