These are the boxes for the system. The woofer box is the first large box I had ever made. I built it in 1998 and have had it in continuous use since then. The M/T box was originally made for the DIY 2002 NE meeting and was later modified for first the 13m/8640, then the 12m/4631. The dimensions are precise, but were I to make them again, I'd do it a bit differently for the woofer, for better ease of construction. As long as the baffle dimensions and driver cutouts are the same, you can make the rest however you care, as long as the volume is correct. There's one cross-brace in the woofer box, the angled piece that passes just above the passive radiator.
There's not much that you can do about the M/T box and still maintain the front baffle dimensions, but the woofer box could be done in a number of ways. The cross-hatch area shown in the woofer box is where I reduced the internal volume by placing the small vertical brace inside. This could house crossover of be eliminated.
The woofer box volume is about 70L. I need to re-calculate it and account for displacement of the cross-brace. The M/T box is 4L.
The first set of curves below are the Baffle Diffraction Simulator predicted diffraction signatures of the midrange and tweeter from the M/T box only. The second set shows the predicted diffraction signatures when the box is placed on top of the woofer box. The latter one was done by extending the M/T sides down to create a baffle equal to the two combined. The width remained 8" rather than 12" as the BDS can't do a box on a box, but it should be fairly close since it will be the step that's most affected for this case.