I installed a big vault underneath a back porch when I was building the house. All walls are 12" block with rebar and concrete filling. A steel deck was laid, then the porch was poured with plenty of rebar. A browning safe door was installed. I put in a french gravity drain around the outside wall. I also put a 1" PE pipe through the wall to drain the dehumidifier. I put rigid insulation on the ceiling and outside wall. I also ran HVAC supply and returns to it. I'm very satisfied.