i transported some 6 ft flowering plants once but they were in pots and soil. I just threw a couple of garbage bags over them, tapped em up and threw them in the back of a ute (you yanks call em trucks I think). Transported at night thru Sydney's inner west with them waving in the breeze with the GF driving behind me.
But yea, even if you buy a roll of cardboard and make your own box up you should be fine.