rgburrill is absolutely correct. The tube does not carry any load and does not need to be strong. It only functions as a servo lead guide. Cut a strip of brown packing paper the length of the wing panel and about 5" wide. Roll it tightly around a 3/8"dowel to form a tube. Slide the dowel and paper tube into the wing and then let the tube "unwind" and remove the dowel. This method works even if you have already covered the wing.