I thought this would have taken an hour or so - max. Wrong.
First I riveted the L-angles to the end ribs; they will serve as braces & connection to the front and aft stab spars. Positioned the end ribs. A lot of confusion as to how they fit. They fit better WRT hole alignment) in the wrong orientation, so dragged out the plans and made double sure of what I was doing.
Re-oriented the stabilizer and re-leveled to ensure everything is hunky-dory. Used a 1x2x6' board to fold-over the top skin (bottom was riveted awhile back) and cleco into position starting from the center and working outward. Riveted with my new riveter I got for Christmas! Works well.
I then turned over the stabilizer to rivet the bottom side of the end rib. These are fitted after the bottom skin is riveted, so had to finish that up.
While inverted I was now able to rivet all of the aft spar upper doublers, caps, spacers, and fuselage mounting angles. Some I was able to rivet with the new riveter. The cap rivets, though had to be done manually due to tight confines. I had to grind down the side of the manual riveter head in order to provide better access to the angle so I wouldn't put the rivets in crooked.
The trailing edge will hold the elevator hinge; this is not to be riveted at this time until the elevator is complete and mated to the stabilizer (black/yellow cleco's in 2nd pic below).