went and got a pizza tonight. here is how i get it home. four straps. first i put on the the two running side to side and they are tightened about as tight as they can be without squishing the box at all. the ones that run front to back are tightened unitl the box won't move.

i think all you would need to do to make this work on a regular rear rack is put a piece of plywood under the pizza box. wouldn't need to be as big as the pizza box, just some amount wider than the rack so it will help spread the load on the box.