Still working on this elevator. Main output today was that I back-riveted the stiffeners to the top skin.
If you are another builder, the way I held the skin open might be of more interest. There are all sorts of blogs that illustrate how to hold the skins open with duct tape. In my experience tape lifts rattle-can primer, so I used the ground wire out of a hunk of 14 gauge romex (14/2-G NM) and poked it thru a couple of 3/32" holes in the bottom skin. This had an unexpected benefit: the loop of wire slides easily up and down the vertical post clamped to the back side of the table. The result is much easier on the skins!
Note that for each stiffener I had to un-clamp the skin, in order to install a new row of rivets and also adjust the backplate position.
Total time today: 3 hours (Went flying too...)