>Much better to send in a trunk
Actually, a foamed box is better than the old trunks were. Much more padding around the harp. The fiberglass trunks in particular were really dangerous.
I don’t know of any freight company around here that would keep the box for you–it must be very different where you’re located. Hereabouts, if there isn’t room, you just have to resign yourself to paying for the harp company to send you a new box if you need one.