Yeah, I'm kind of confused too. The typical cantilever hanger is what, 2 mm thick? So I don't know how you get from that to accidently cutting off 2.5" too much. Oh well.

At any rate, the easiest option is the extender.

Next easiest is buying a replacement fork and having it powder coated.

Third is brazing in an extension. Drilling out the steerer is a very bad idea. The insert needs to be turned to fit. Have the person doing the brazing use silver instead of brass, as the cooler temperatures necessary for silver are less likely to compromise the existing joinery and paint. Many/most welders do not know how to braze thinwall tubing and might make a mess out of it. You really ought to find a framebuilder for the task.

You might also look into using a stem with a built-in cantilever hanger.