Refitted starboard wing and am pleased with the line achieved.
Wedged/glued bits of 3/8" H45 into the gap, (top and bottom), spacing from the achieved line with 1/16" plastic packers. Hot glued the packers, urethane glued the foam.
Experimented with some sort of backing plate for the rib-to-rib gap, without much success. Want to minimise the amount of RTV in this joint. Tried some scrap cured 1 x BID with glue on the face and threaded with fishing line to position. Just made a mess. May try some expansion joint foam next, glued to the inside of the wing rib flange.