As I have a few wires between the subpanel and the firewall that would be only accessible from under the panel, I decided to install two access panels on the forward skin. This modification is not on the plans but has been tested by a few builders.
The access panel being thicker than the skin I have to install a shim between the skin and the reinforcing ring. The picture shows part of the shims.



I then cut adn drill the reinforcing ring (square).



The same from the other side.



And with the panel installed.
I still have to sand, prime and rivet. I also have to finish the second panel.