If the only box you have available is too large for the piece of pottery, it is very important that you fill the empty space in such a way that the pottery and its cushioning material cannot move inside the box. As you fill the empty space, add material so that the pot remains as close to the center of the box as possible.
If you have sheets of foam board available, it can be cut to the right dimensions and layered to fill up the empty space. Other solid forms that have a bit of give to them can also be used.
In the example photo, you can see that the pot is packed in a Styrofoam container (the cushioning material), with blue box on either side. Both these blue boxes were empty. I filled them with wadded plastic grocery bags to stabilize them, then taped them closed.
Once inserted into the main box, they keep the pot and its cushion from moving side to side. The empty space at the top will be filled with more wadded plastic bags.
