There's another large low horn out there - the LilWrecker. This one is a more complex build than the LilWrecker. Between the two, the output's about the same, within a dB or two and a Hz or two where it matters.
Lots going on in there, but it was designed to be buildable with basic hand tools (circ saw and guide, router, saber saw, drill. and screw gun).
Bracing is NOT OPTIONAL on this cabinet - it is absolutely needed. A single brace in the middle is a good start, make sure the braces are windowed as in the photos.