Personally, I would find a box a good 4 to 6 inches bigger all round than the lens box, and then pack it round with polystyrene filler, that's how my lenses have arrived from shops, and how I post them.
Check also that the courier you're using actually insures camera lenses (you are sending it insured I presume?) as many don't. Those that do will often only cover them for loss, not damage.
Interlink is a company I use, they will cover camera lenses for damage also.