I haven't seen one of those pods before. Where is it?

Something like the Abus type locks (link and pic below) would be best for locking the latch on the pod. If sized correctly they greatly reduce the space available to get a set of bolt cutters in, relative to a normal padlock, and are very hard to cut. I would agree that a u-lock might fit and would be preferable to your cable or a typical pad lock.

The idea of using your u-lock and/or cable underneath the pod as a second line of defense seems to be very reasonable and probably fairly quick & easy on a day-to-day basis.