Cut 4 F-637C angles from 3/4" x 3/4" x 0.063 angle stock.
Measured seat back skin to determine rivet spacing (photo 1). I then used those measurements to make a template out of a strip of 0.032 aluminum from the wing trim bundle by drilling a #40 hole at each location. I then used the template to mark one of the F-637B upright angles by just drilling a shallow dimple through the template holes. Measuring these dimples showed the bottom most hole about 1/16" too low. I'll relocate that dimple before final drilling the upright angles on the drill press.