I'm also doing the entire outer shell of the fuselage with coffee filter paper and WBPU. it's very hard when dry and light. not quite as strong as glassing with epoxy, but very good for what it is.
here's how my wing box is constructed: the wing is in three pieces before final joining, the two outer and the central wing section. I knew the thickness of each wing rib, so I was able to make a template of the two wing spars, as if looking from the front and rear. these are your vertical surfaces of the wing box. decide what thickness your balsa facing will be (I used 1/8 inch) and cut spar out of foam to suit. this balsa facing will give the foam incredible strength. make the rear spar the same way. your foam wing ribs link the structure together, but I remove every other rib, to allow the wing to flex somewhat. this extra space also allows for internals such as flap mechanics, etc. remember to make the rib in the nacelle pylon location out of ply or a composite sandwich of foam and two balsa layers. I join the outer panels to the inner panel with a male/female join and epoxy. the join is at the ply rib for the engine pylon for extra strength in that location.
just thought I'de share how I did it.