Do it like this:
1) Make a disk out of aluminum or plastic of exactly or very close to the diameter as the inside diameter of your smaller tube.
2) Drill a hole diametrically through that disk at slightly a bigger size than the holes you are planning to drill through the side of the two engaging tubes.
3) Make two pins with a stepped shoulder where the larger diameter is equal to the inside diameter of the hole you drilled through your disk in step 1. Make each pin about a 1/4 in length of the diameter of the disk you made in step 1. The smaller diameter of the pin should be equal or slightly smaller that the holes you're going to drill through your tubes and with a length equal to or slightly longer than the sum of the wall thickness of the two engaging tubes. Round the smaller ends of the pin.
4) Find an appropriate size of spiral spring with an outside diameter slightly less or eqaul to the diameter of the hole drilled through the disk in step 1
5) Assemble the parts inside the hole in the disk. Place one pin inside the hole in the disk, rounded end facing outward. Insert the spring in the hole followed by the other pin with the rounded end facing outward too. You should now have the spring sandwiched in between the 2 pins inside the disk.
6) Compress the pins and spring inside the disk and slide the whole assembly in your tube until it lines up and locks with the hole in your smaller tube.
You should be able to figure out after these steps what to do about it next.