With 1/4 x 3/8 balsa cut the needed braces for the ply side. Hard balsa grade recommended and make tight joints. Leave the bottom open for the landing gear block along with the are for top and bottom longeron.
Braces installed and now ready to attach to the fuselage frame you made above. IT critical that you get it aligned perfect.
The finished assembly
Now repeat the left ply side remember it’s a mirror image of the right except the door opening.
Reminder the left ply side is a mirror image of the right except the door opening.
Lay both finished assemblies on top of each other and they should match perfect.