The plans I distribute are for 10ft3 cabinet, before internal component displacements are even considered. And you can get excellent performance from a 2245 using as little as 6 cubic feet. PiAlign suggests 8 cubic feet, tuned to 30Hz. Anything from about 6 to 10 cubic feet gives very similar performance, with the cabinet tuned to 30Hz. So you can really take some liberties with your cabinet shape and dimensions, as long as it falls within these limits. I'd consider anything in that "window" as being PiAlign'ed, since it acts fundamentally the same.About your crossover, I'd recommend you go active when impementing subs. Many amplifiers are specifically designed for this purpose, and you can simply set the crossover point.