The horizontal stabilizer is complete now. The bottom half of the skin has been riveted on.
Despite the spars and tip ribs lining up perfectly, the two inner middle ribs were mis-aligned by just a hair too much, and they wouldn’t shift any farther. Rather than create an oblong hole in either the skin or the rib, I opted to drill out to a #20 and use A5 rivets instead of A4.
Update (Wed 10⁄20): It’s now hanging safely above the shelf