Riveted nut-plates to a couple of reinforcement plates. These reinforcement plates were then riveted to the bottom skins. These nut-plates will allow a second plate to be screwed to the skin, rather than being riveted, making it removable for inspections, maintenance, etc.
The contraption in the picture is a little setup for "back-riveting" Rivets can be squeezed or they can be bucked. When you squeeze a rivet, the end that sticks through the metal deforms outward and downward forcing whatever you're riveting to stick together. When you "buck" a rivet, you hold a dense piece of metal like steel or tungsten (called a bucking bar) against the end that sticks through the metal and on the other end you hit it with a rivet gun. The rivet gun hammers the rivet into the bucking bar causing the same deformation you get when squeezing. Sometimes it's convenient to buck the rivet upside down. This is back-riveting. I put the rivet into the hole and tape it in place. Then I put that end against a flat piece of metal (called a back-riveting plate) and then hit the other end with the rivet gun, using a special attachment. The same deformation happens. You just rivet by whatever method is most convenient given the circumstances. Tonight, I had tried to squeeze the rivets holding the reinforcement plates to the skins, but was unable to have everything hold still to do it properly (even had to drill out a rivet that was set improperly). So I got out the back-riveting tools and went to town.
Also notice in the pictures a new make-shift table. Too many things taking up space and I like to back-rivet using that small table on the work-bench. So I laid some plywood over some saw-horses and moved the left elevator stuff over to it. The next steps also involve a lot of back-riveting so I'll leave everything as is for now. Eventually, when the back-riveting is complete, I'll be moving the left elevator back to where it was on the work bench.