Properly wrap the bow and use enough tape, the triangular boxes will work. I use shipping tape on boxes, duct tape or even better the Gorilla brand duct tape to seal off carpet tubes. I have used the triangular boxes in the past with no damage.
Newspaper is cheap, and works great to wrap the bow and fill in the gaps. Be sure to put extra on the tips, and make sure they won't push through. Again, used enough paper and use enough tape! A dollar or two worth of tape may save a lot of damage from happening. Cheap masking tape is fine for securing the newspaper.
I always insure every bow, and only ship via the USPS. Several hundred bows shipped all over the country and all over the world, and to date none have been damaged. Sometimes the package looks rough, but the bow inside has been fine.
Chad