This is actually intended for full range 3-way tower speakers, the cabinet may be built in two pieces that joing together with a tounge and groove around the perimeter and sealed with a gasket, the weight of the top portion should be enough to keep it sealed with out any fasteners to the bottom half.
The concrete material we would be using is actually spraycrete, it is a very brittle form of concrete and our preliminary test panel resonates at a very high frequency so I don't think resonance will be much of an issue, but it reflects a lot of sound back from it. The walls would be 3/4's to an inch thick, and reinforced with wire mesh. Definately need some sound absorbing material all around the interior of the cabinet. The spraycrete isn't as pourous as poured concrete so it may not even require any sealer on the inside.