Welcome to the clubhouse. Olde Lowe sells a 6" brass cone here - http://smokehouseguitars.com/blognewsroll/?cat=38 - for $25 - not at all unreasonable, and everybody here who has 'em loves 'em. As far as "plans" go, most folks here don't use them - for the most part, when you have to work with the box that you have, a plan becomes pointless. Pick a scale you like, find a mounting arrangement you can live with, and you're off to the races. This is a pretty helpful bunch, so toss around the questions.