Most cities have stores that sell used boxes. If you find one or two that work you can line them with sheets of Styrofoam from big box stores like Home Depot or Lowes and cut them to size to firmly enclose the speakers at least 2" on each side. Be certain to insure the shipment. I've done it with speakers, amps and subs and all but one arrived without incident and the one that had minor damage was covered by insurance. Also, I'd avoid UPS (pronounced "Oops") if at all possible.
