A little bit of custom hardware.
X-mounts screw into the plywood firewalls. Firewalls will be just glued onto the motor pods with Gorilla Glue, two toothpicks will drive through each firewall, glued into the foam, with CA between toothpick and plywood. CA soaks into the wood really well. Custom servo horn for two rudder control rods.
Way back I bought one 4-pack of the blue 9g servos, because I don't know why.
(All the rest I have are 5g or less.) They didn't include servo horns that would give me the angles I want. Then I found that each manufacturer uses a different number of teeth on the horn-to-servo connection. So I made my own so I can get the angles I want.