I've used Cubo 12's with extensions myself, personally I like extensions that are at least the same depth as the Cubo itself (in this case roughly 50 cm) or more. They extend the usable lower limit a bit (about 5 Hz), mostly they give a very decent increased output between 50- 80 Hz. To really drop a lot lower, the part before the extension needs to be designed the right way, which is not typically what you will find in any cabinet that's designed to do it all on it's own. Therefore I would suggest Cubo 12 Mod which drops at least 5 Hz lower than Cubo 12 in exchange for less output in the 50 - 80 Hz region, together with an extension it will definitely hit your lower target, plus the extension will make up for any loss in the 50 - 80 Hz region.
However for trance you might find Cubo 12 (with or without extension) already going low enough, Cubo 12 Mod with extensions (and Cubo 15) definitely will.
the model/cutlist assumes 15mm ply, and 3mm blade kerf, cut from a single sheet of 8*4 (actually it's 1800 x 1222)
if you go to "cubo 12 (layout)" tab, and double click "auto layout 1" on the left, you can adjust to match your job (spacing = kerf)
i'm pretty new to the software and not totally sure what will happen to the drawing if you ammend the autolayout, so make sure it's right before you cut anything!
i've backed the file up so it's not catastrophic if you break it but please let me know if you're making changes so i can keep an eye, and if anyone sees any problems / mistakes please let me know!
You are correct of course - I have missed the big brace!
I actually made the real bin first and then the model, to be part of a larger model of the bicycle trailer system that my cubo is destined for... (will share later in case any one interested), so my model and particularly cutsheet have not been 'idiot tested' by actually being built from a print...
Will reduce driver aperture and add brace "b", also annotate the angled cuts!