The result is OK but not very neat. At least it sits perfectly flat so it should not affect me when I am adjusting tweak.
First photo is of the preparation I did before I secured the main chassis stiffener. Oleg advised to scratch the surface of contact between chassis and stiffener in order to prevent any movement in case of a hard hit.
Second step was to install the stiffener. This is the long option which runs from the front all the way to the back. It also means that the servo holder's position is pushed to the outside as the central option is now covered by the stiffener.