When I had to ship a UPS back to APC without the original box, I took the actual measurements, bought a significantly oversized "double-walled" premium cardboard box from a removals company as well as a huge sack of polystyrene pieces (not the easily compressible starch ones but bits of expanded polystyrene cut up into chunks). Once it was appropriately cocooned, I then taped it up real good and made sure it was insured when posted.