Upside-down U. This will prevent moisture from being trapped by the rib.
So, if we could revisit this: if the ribs on the stabilizer and elevators are open side down, how does the gusset on the bottom side rivet to the rib? It seems if the rib were turned sideways that the gussets could be riveted on both sides. Also, the drawings for the stabilizer and the fin indicate "flat side" of the root ribs. I assume this means the rib should be open end up on the fin and open end facing outboard on the stabilizer. Correct? Thanks.
Jerry, as best I can tell you have it backwards.
Face the middle part of the U (closed side) toward the root of each horizontal control surface.
Now, imagine you are a drop of water inside the vertical control surfaces. We don't want you to get trapped so the closed side of the U faces up. This always gives you a flat on which to attach each gusset.
In the photo below the top of the rudder is to the left.

If this was the right elevator, the root end would be to the left.
No need to over think this.
