I made huge strides in my ability to shrink the aluminum, but it wasn't enough.
It looks like I made it to about 70-80 degrees or so, whereas the end-point should be around 110 degrees, if I recall. That may look and sound like I'm getting there, but if you think about this in terms of the reduction of length or increase in thickness around the perimeter, I couldn't be more than half-way. This piece here is about 5 hours of work, and up against the limits of what I think is doable, even with lots of patience and practice (i.e. if I could avoid big catastrophic tears like the one shown here).
It may or may not be possible to go farther with more annealing cycles, but this just isn't what I call 'favorable'. Even if I threw another 40 hours and another sheet at this problem and ultimately succeeded (but I think I would not succeed), but even if I did, there's a very real chance I'd end up with poor strength in the fatigued, work hardened corner. Fiberglass must be a better route.
So here we are, corners snipped out, two front skins riveted in place. I'm still not sure why these ended up a few inches short. I'm going to end up with a little strip to patch up that gap, but at least it will be out of sight.
First the bottom seam, then the side. Clamp, drill, disassemble, deburr, construction adhesive, reassemble, clamp, rivet.
This rib is also the lip around the front of the box. It's made from .75x.05" aluminum right angle, like the other ribs. I used a jig-saw to define and serrate half of the right-angle, then broke out the tabs and filed, and bent the corners around some of my .5" steel rod.
I must be behind one photo, I did finish up the glue and rivet for this rib last night.