It's not about how inert it is, but how stiff it is since it's a lot easier to push the cab's Fs above the intended BW than below it to ensure it can't be excited. I mean if you want to make it inert enough to push its Fs below the intended BW, that's fine, just understand it needs to be built out of many inches thick concrete or similarly massive material and damping TT rumble will probably require walls built like concrete suspension bridges to get such a low Fs damping.

Others have gotten close enough with stone slabs (slate, marble, etc.) over a plywood construction, but they were small speakers with a fairly high Fs, not the low tuned, relative behemoths we ideally need; so using at least 19 mm no-void Baltic or Apple Birch combined with some bracing to tie all six sides together to keep it from 'breathing' will work fine, and while not exactly light, at least it's movable by most folks with a cheap hand truck if there's not a lot of stairs.

Making it some form of TL allows less bracing to boot since its high aspect ratio places most of the resonant pressures along its length, so not as much cross bracing required. Factor in that the bigger the cab, the lower the pressure nodes and very little bracing is required, mostly in the form of ridge boards glued in to break up the large panel areas into a lot of very high aspect ratio strips with a cross brace to do double duty as a motor support.

All that's left then is reflections off the back of the driver, which is easily handled by the standard top, one side and back being covered with insulation to 'taste'.

GM