I was never really sold on the whole Ptolemy concept, I always thought they'd go for something more similar to a modern container ship. Rather than huge pods being dragged around I'd go for a flying warehouse or a platform that holds the containers snugly. The pod seems redundant to me, a wrapper around a wrapper to allow a specialised ship to carry goods. Why not just have that specialised ship just carry the containers directly?
Did I mention your models look superb, by the way? Lovely texturing job too.