I bit the bullet and cut "slots" for the doubler on the inboard R2 ribs. The instructions called for "slots." You can see how I interpreted the instructions in the picture below. After doing that, it was rather simple to install the doubler. However, I should point out that I had to shorten it and then cut the ends to match the dihedral. Neither of these tasks was mentioned in the directions or plans. Then I glued the plywood plate to the front of the doubler.

Unfortunately, my favorite task now awaits... LOT'S-O-SANDING!