After taking some measurements, I found out why the outboard upper skin doesn’t fit will. The pitch of the holes drilled in the skin is just a little bit off compared to the pitch of the holes drilled into the rear spar – over the length of 43 holes, there is a difference of 1/8″. I ran into this on the left wing as well, and after consulting with TAF, the solution was to elongate the holes a little using a file and a reamer.
After I tried elongating a few holes, the skin sits flat and looks good. Next I can take the skins off and start pinning the ribs onto the spars.