BMatthews

The picture showing the joined trailing edge is fuzzy around the trailing edge becuase it focused on the clamp so I can't see the problem you describe. But doing it the way you are with the inner panel rib upright and the outer panel rib with all the dihedral angle is one reason why you're having trouble getting the trailing edge to fit smoothly. The angle needs to be split evenly between both so the parts fit smoothly. Done the way you did it you're bound to get a step in the upper surface of the trailing edge where they meet. See the sketch below.

In addition to the purely geometrical issue a couple of construction technique factors can come into play. First is that if you block sand the wing faces in prep for doing the joint in an attempt to get a nicer fit you need to support the ribs better so they don't just push inwards and not get sanded enough. Also you want to use the edge of a table or custom sanding board to guide the block so the trailing and leading end grain sections don't get abraded more than the face grain of the rib. The end grain will tend to cut easier so it's important to watch for that. You can make it all work out easier by using fresher sandpaper so it cuts easier and use minimal pressure so the grains of the paper can cut without being forced.