To better define what I was saying in my earlier post, the point I am trying to make is this. With what you are trying to do you have to look at the whole assembly as a whole.

The top of the cabin is going to have to deal with the total of forces I described each wing root and strut attachment point will have to support 31.25 lbs. of force and that is what you have to try to picture.

Not all of that force will be in one direction at any give time with the exception of your cabin ceiling or roof having to deal with the compressive force most of the time. If you make you cabin roof /wing sheathing over the cabin out of 3/16 to 1/4 inch stock which is fiberglassed on both sides this will have a lot of strength under compression if you run you balsa from side to side. So all you will have to worry about is having your mounting surface deal with the shear force caused by the wing root by making this mount a part of the door window frame you will be distributing this stress down the side of the fuselage

By making the root rib out of 1 inch thick stock all of the load can be taken up by the tubes that you use as sockets that slide over the rods protruding from your fuselage sides and this will give you a lot of area to distribute the load to the Spars and sheathing you use for your wing. So as you can see I would sheath at least from the root rib out to the firs or second set of ribs. You might want to fiberglass the wing root assembly also.

Now to secure the wing to the rods all you have to do is drill through you tubes/sockets and rods in the center of each rib thread the rod to accept a machine screw that you run down from the top of the wing through the socket tube and into the rod and back out again so it extends toward the bottom of the wing place a nut on it for a safety and you wing is secure. You can recess both the heads of the screw and the nut below the surface of the wing for clean airflow.