There may already be a hardpoint in the wing somewhere for a retaining screw. If there isn't, cut a small hole in the bottom of the wing, over the main tube, at the outboard side of the last rib the tube passes through. Glue a dowel or hardwood block into the hole and against the rib. Drill through the dowel/block, tube socket and tube, tap the wood and put in a socket head screw long enough to engage the tube. Epoxy the other end of the tube in place.

For easier transportation, I would glue the tubes into the outboard sections and screw them into the center section, but I can't tell from the photos whether the structure would allow that. You don't want to screw them into both sides because aligning them each time you assemble the airplane would be a b***h.

Jim