My aluminum order came in, so I decided to work on fabricating the rib doublers from some .050 thick 2024-O aluminum. Here's a sample using a ~.050 bend radius. The outside is ~.100 bend radius which nests nicely in the .093 bend radius of the ribs.

Cut rib blanks and calculated the bend line offset.

After notching the corner and making the first bend, it occurred to me that the second bend needs to be pretty much an exact match for the rib....

...so, I nested the doubler on the outside of the rib and traced the bend line along the existing flange of the rib. It's good that I did this as the bend was non-perpendicular by probably .030 over the length of the doubler.

...but using this technique, the doubler matches perfectly.

Used the same approach to do the outboard doubler on the left side.