Stated above is the practical solution.
As an example: The damaged tunnel area is 7 inches long, and extends laterally into the flat portion of the floor for about 2 inches on each side of the tunnel.
From a car in a wrecking yard a matching piece would be cut that is about 9 inches long, and 3-4 inches out on each side.
After trimming to give an overlap of about 1 inch all around, the piece can be attached on the inside of the tunnel/floor via riveting and bonding with structural adhesive.