There is no reason why you couldn't build a rib-shaped shim either out of balsa or ply. Cut the shim rib out of material that is as wide as the gap at its widest, then sand it into a wedge. This is easily done if you build a jig to sand it on. It doesn't have to be perfect. Slip the shim in, and fill any gaps with epoxy and microballoons. Easy! I did this to take the dihedral out of my trainer wing. Be sure to glass the joint when you are done, and your are good as new.