If I was upgrading, I would certainly go for a plastic enclosure (see animal plastics or constrictors northwest) with some space to it. For example 4' x 2' x 1'.
I agree with this for two reasons: (1) When a BP is swallowing prey they will often "rise up" to assist in swallowing a solid 8-10" or so (depending on snake size), so additional height is helpful to them in this respect. (2) With a tub, it probably doesn't matter so much due to it being top-open, but for a side-open enclosure I really like some extra height 14-18" to be able to maneuver around inside the cage when cleaning or moving the snake. The extra height makes a world of difference.
The christmas tree tubs reference above are quite spacious and could be used for a ball python, they sell them at walmart or the container store.