Unless there's something I don't understand, there's no magic in this. If you want a shorter, deeper box, you build according to Giskard's dimensions. Otherwise, use those of the 4645. In either case, you'll want to use heavy material and massive bracing. For more alternatives, and construction suggestions, check out this article from the library: http://www.lansingheritage.org/html/.../1983-subs.htm.

Either driver (2245 or 2242) will work an any of the boxes. The JBL Tech Note says 2242 replaces 2245. They have done so in their product offerings, so I take them at their word. Both require EQ to reach 20 Hz. BUT, it doesn't matter. In home use, room response augments the low end to obviate the necessity of EQ.

You want more, build 2 or 4 of them, and mutual coupling will push the response further down there as well. JBL recommends using arrays of 8.