The Trick is to BUY one first, then re-use that box!
Also, while it's kind of expensive, you can buy a 54" X 20" X 8" box from The UPS Store, or where ever. It's $15 - $20 for the box, but it will let you pack and ship safely.
Some tips I would offer:
Pack the INSIDE of the Case Best! Wrap the neck in a piece of Bubble Wrap and fill any space that will allow the bass to move at all inside the case. You should have to use a little Force to close the case to squeeze the Bubble Wrap inside. Now shake the case Vigorously. Is there any movement? If so find where it is moving and pack some more "Stuff" in there!
Place a layer of Bubble Wrap in the bottom of the box, and set the case in standing up. I like to add something Solid, like StyroFoam, between the box and the case sides to restrict movement. But a Roll of Bubble Wrap works well. First pour in enough "Peanuts" to fill the box about 1/2 way. Then add your Foam/BW Rolls to keep the case centered, and then Top her off! I like to add another layer of Bubble Wrap on top to keep the peanuts in the box.
Tape it Shut with a single piece of tape and Shake Vigorously! Is there any movement? If so get rid of it! Once it is in there Solidly, Tape it shut Really Well!
I also place "UP" with Arrows stickers on all 4 sides, LOTS of RED "FRAGILE" stickers, and "HIGH VALUE" stickers on all 6 sides. I print them using WORD, and just tape them on.
Also, IF something happens to the package during shipment, UPS, FEDEX, USPS, DHL, etc... will ALL deny your claim. Do Not Give Up! I have never had an appeal denied. They are just trying to keep costs down, and it's Business. After you get the box back figure out how THEY damaged your package and present the facts to them calmy and clearly and they will settle up with you!
BTW, shipping in a Gig Bag is a LOT Easier!