I realize this thread is old, but, I found a pretty slick deal of $20 on a 36x48x24 laptop cart. Schools have all been "1 to 1" with classroom laptops for long enough now that the beancounters have approved replacement of their carts for new "fancy" ones. Good for us, because a large steel box with big casters is expensive, normally. Take the shelves out and you have like $200 (at home depot pricing) of sheet metal to cut up, as a bonus. Govdeals, fb marketplace. I'm lining mine with foamboard insulation and concrete board to be my battery enclosure.