Some of you sound confident that it's no big deal. I'm still nervous about it.
How about a cardboard box, styrofoam boards on all four sides, and the power tubes boxed in bubble wrap and left inside the amp?
...and maybe enough packing tape to practically seal the box.
Is there a safe way to pack a mark iii without a road case? I suspect the answer is no. I can't imagine a +70lb amp having any respect for any kind of packaging material.