After a couple of weeks of life got in the way, I finally returned to this vertical stab top cap. I sanded the surfaces down and filled pin-holes. I also filed recesses into the fiberglass plug's shoulders so that they would fit around the two rivets holding the stab skin to the rib flange. With the fit right, I final-drilled the two rear-most holes for LP4-4 pop rivets in accordance with the call-out. Once mounted, it looks great, and it's ready for paint.
Dried, sanded to fit around the skin rivet shop heads