I get boxes from Uline and filling around the hardcase with paper or something. Not too tight because I want some cushion between the box and the case in the event of a drop. Inside the case, I stuff everything with soft-ish packing paper so that sucker cannot move around, including making sure the neck is well supported. I also leave the guitar tuned to pitch and put the paper between the frets and the strings to avoid damage there. I've shipped about 4 guitars (most with tiltback headstocks) this way without any issues.