Hi Tarquin - looks great so far!
This is a little late (for this build), but I sheet the top of built-up wings (MK kits) before joining them. This adds a significant amount of ridgidity. I then jig the two halves; joiin them and add the bottom sheeting while the wings are jigged. I also add the fiberglass center section reinforcement. When that has cured I can take the wing off the jig, flip it and add the top fiberglassing. This method has worked for me for many wings.
I would carefully jig your assembled wing then add sheeting (top or bottom depending on how you jigged it). At the current stage, the wing is quite delicate as I'm sure you are aware. when completed it is very strong and light!
Keep us posted - very enjoyable!!
-Will B.