I have made a few powder horns and never went to the trouble of boiling/soaking and shaping. I figured the originals weren't either. To reduce some of the dust and to also speed up the smoothing process, try using broken glass. Beer bottles work good for this. Of course if you have a cabinet scraper that will work too, but they don't taste as good as the beer does.
Also when I install the plug, I trace it on the wood blank similar to what the first link showed, then I use my router to cut it out. I set the router depth to about half the thickness of the wood blank, and then cut out the outline just on the inside of the line. Some rounding off still needs to be done on the part of the wood that fits up into the horn, due to the curvature of the horn. When your done, if the horn is square, and your router cut is square, you end up with a nice tight seam bewteen horn and wood. There is also enough wood inside the horn to drill into and install decorative pins. By the way I use copious amounts of epoxy to ensure a good seal. If I do my part right it does not show up on the outside.
