Not really sure what you are describing. Like da rock said, if you have a recessed wing mounting block, and you are trying to fill the area between that and the top of the wing saddle, then use a balsa block, and sand it to the shape of the wing saddle. You will have to drill through it and through the ply mounting block for your bolts.
If you are talking about the area between the bottom sheeting and the top sheeting of the wing, at the TE where the bolts go through, then that too can be balsa filler, sanded to shape to keep the sheeting from collapsing when you tighten the bolts. in addition, you may want to use some thin ply on the top sheeting of the wing to further distribute the pressure of the wing bolts.