From what you are saying it sounds like it used an aluminium tube joiner. Is that right? If so I strongly suggest you find the original or use a proper aircraft aluminium tube. The stuff sold at the hardware stores looks nice but it's about as strong and springy as over cooked pasta. Real aircraft tubing of the proper sort and proper heat treatment spec is night and day different.
It is odd for a model glider to use larger diameter aluminium tubing as the joiner. What is the inside diameter of the receiver tubes in the wings? Perhaps there's something else you could use?