I was surprised to hear that the tube jig for building didn't go that well. Building inverted using a forward box works well even if the box tapers aft, though probably heavier than formers only. I built an original design last winter that way. As you are thinking, it was built inverted to the stage of the forward round former and upper fore and aft turtle decks and hatch but can come safely off the board by those with no concern for staying aligned.
I'll offer my build pages for your reference.
http://pages.suddenlink.net/arlyn/30cc_ss.html