The 3/8 square trailing edge is attached to the ribs with a butt joint then you have to sand it to conform to the wing contour. You might want to consider marking the exact location for the ribs using the plans then cutting a kerf in the TE into which the rear of the ribs will fit. It doesn't have to be much perhaps 1/16 inch. That would make it much more sturdy and withstand the sanding better.

Incidentally, my wood for the fuselage is like oak and the sheeting for the LE and TE are so weak you can put a finger through them.