If it weren't that Paba asked for an easy-to-build bass reflex box, I would have definitely suggested the 12π to augment the bottom octave. I'm not sure he'll need or want subs, but if so, they're available at various price/performance and DIY skill levels.
12π basshorn subwoofer
The four π sounds pretty good all by itself, but if subs are required, there are a few good smaller (and simpler) cabinets that would work very well. The 12π basshorn isn't any more difficult to build than any other folded horn, but any basshorn is definitely more complicated to build than a bass-reflex box. Something like the (10ft3) four π-18 subwoofer or the (4ft3) three π subwoofer might be more viable, if subs are required.