Go to Lowe's et al and ask for a water heater/weed whacker/etc box. They get some big cardboard there. Cut the ends off and wrap it around your rifle. Tape as necessary.
When you go to ship, they may measure length, width and height. They always come up with a very small number when a side is just a fold. Most of the time, you save money when shipping the above compared to an intact box.