Proof of progress...
I had a few minutes after lunch here in my home office so I went down to the basement to take some photos.
3 of the 4 DIY Soundgroup 4ft^3 sub boxes complete...
...and the fourth just unclamped and now waiting for its baffle installation.
Just for kicks I took one of the two custom rear sub boxes and threw it into the recessed back box I created. Now as dumb as this sounds, I quite literally cut a piece of scrap 2x4 to the exact width of the subwoofer plus 1/2 (leaving 1/4" gap on either side when installed) to make sure my inside back box dimensions were consistent. I'm not sure how it ended up like this, but the sub was more like an exact fit with barely 1/8" to spare. You could feel the 'cush' of air rushing out of the back box as I slid the sub into place.
Here you can see the incredibly slim gap along the vertical edge....
For the height, I left just enough room for the subwoofer to have the beefy rubber feet and accommodate a right angle Speakon connector. Here you can see the remaining gap above the rubber foot is just barely enough to accommodate the connector when the sub is shoved into position.
And finally, a pic of the installed sub box, just for fun.
Rear acoustic treatments will be 4" deep, so this low-profile installation will allow 2" acoustic treatments on the surface of the subwoofer where-needed and allow about a 2.5" gap between the subwoofer surround and the back side of the acoustic fabric to allow for XMAX without the driver 'thwapping' against the fabric when everything is operational.
Next up is lining and filling the boxes, plus preparing for final front baffle glue-up and installation on these rear subs and the two 24" subs.