That's easy. You get a hole saw of the correct diameter, with the finest teeth you can find, and you mount it on an extension and ram it down in there, and saw it off. There are bunch of things you can do to make this work better or worse. Cheaper or not. But if you are asking this question you probably can't pull it off. What you probably could manage is spinning it on dril first, and holding it against some abrasive to get the outside set off to minimize the chance of tearing up the tube. Use lube, and try to get it started as near to the wall as you can because once you get a kerf in there, you are going have a harder time starting it anywhere else.