Well, it looks like the easy part is done. I am about at the same point of build that you are . I was thinking of building a fiberglass/epoxy dome cover to install on the back side of the firewall with a small drain hole to drain the carb spit and that would seal up the velocity stack while allowing enough airflow to the carb. The dome would be large enough to drill at least four 1/4" hole through the firewall to give additional airflow.
I am not as scientific as you but my hole is about the same size as yours. The .64 cent question is how to install a good strong top cover to the motor box that will give added strength but will allow it to be removable. The reason I want it removable is there is so much room inside that could be used to mount the engine ignition box and other items that cannot be mounted on the outside due to the cowl fitting so tight.
I guess I better put some other projects on hols and get back to work on the double trouble.