Summary

I tried a small modification in connection with the wing attachment. Instead of inserting a thread for the M3 screws into the ends of the 1000 mm carbon tube, I glued in a short piece of an M3 threaded rod (25 mm long). This then protrudes about 10 mm from the carbon tube.
I then screwed a dummy position light with an M3 internal thread (press-in nut M3x6x6) onto this threaded rod and used it to brace the wing. This worked quite well. I then screwed a dummy position light with an M3 internal thread (press-fit nut M3x6x6) onto this threaded rod and used it to brace the wing. This is easier than inserting an internal thread into the carbon tube.