Your method is similar to Bob Freeman's except he shortens the outer tube from the bottom. He has a very informative Flickr album showing all the steps.
He uses a drill press as a sort of lathe at one point, to prep the plug at the bottom for gluing back in. Clever.
If you don't have a drill press, this step can be done by chucking the part in a hand drill, then holding the hand drill in a bench vise so you have both hands free. Material removal is done with a hand file, held against the spinning plastic, can be done very precisely. Remember to push the file through a full stroke; I see people holding the file still, but then you're only using one tooth (or a few), and that part of the file loads up and stops cutting.

BTW that drill-in-a-vise is useful for a lot of other things, a "poor man's lathe" — it can be used for lots of bike hotrodding tasks, like reducing the diameter of a bolt head. Combined with a hand-held belt-sander, can do cylindrical or conical grinding. Like sharpening the end of a thinwall tube to use as a leather punch. I just did that one the other day, drill-in-a-vise, even though I have a lathe, I just don't like getting abrasive dust on the precision ways if I don't have to.

Your method, tapping new threads at the top, doesn't need the drill or drill press, and is probably faster.

Ooh, maybe shorten the outer tube at both ends, so the Silca logo remains centered? Haha just kidding...

...or am I?