The copper tube will need to be small enough to fit inside the gas tube and the hole in the reciever.
If you use a small length of tube you should be able to have it sit just at the joint of the gas tube and the receiver like so:
Then its just a matter of reinforcing it in such a way so that the pipe doesn't move. Ideally it should just be a friction fit.
Measure out the inside diameter of your cocking tube and buy about a foot of appropriately sized copper pipe. Then experiment a bit before you have to cut away anything.
If you do things right you should be able to keep a working cocking handle for the gun.