i would just get a rubbermade storage bin and use they are taller than the storage box and you will need the extra height for your sump to hold water in case of a electric failure so when the tank drains down into the sump you wont flood your floors. the holes you drilled in your drawers look fine you dont want the water to stand you want it to filter and drip out. if you need more flow you can always make the holes larger.