Is there a Co-op department store near you? I get big boxes for side saddles out the back there when they chuck out the washing machine and fridge/freezer boxes.
If there is a local storage company near you - SafeStore or Big Yellow Box Company they have perfect boxes for sending saddles. That's where I went after selling my daughter's saddle on ebay - also bought a roll of FRAGILE tape which I wrapped around the box many times - no excuses for damaged goods at the other end