The hole has to be about twelve feet wide, by sixteen feet long. Depth is up to you. The deeper, obviously, the better. Save what dirt that comes out of the hole, as it can be used to cover your box. Based upon experience, in packed soil, with six inch rock, a four feet deep hole took me four days of unforgettable hard labor.
Many hands make for light work.
The simplest waterproofing would be by using landscape plastic. Six mil would be the best. But if that's not available, the heaviest you can get is better than none. You are going to wrap the entire box in plastic, then fill in the hole around the box and cover it also. This is why deeper is better.