I've used UHaul trucks. In the old days they had wood floors and if holes didn't already exist from a previous customer, you could discretely drill a few 1/4" holes and dirty them to look old before returning the truck. Nowadays, they are all aluminum floors and you need to tie to the walls.
For halfway across the country, carefully tying off will work. Or you might consider putting the bike on a 4X8 of 3/4" plywood with carriage bolts that you could tie down to. The entire 4x8 will slide around if not tied tight into a corner of the box.