I agree with prathmann, bungies should always run front to back, never side to side, but unless you have a very long rack or a very small pizza, the box will hang over the back and still be subject to forces that are likely to cause the box to fold.

A task best left to the professionals. Most pizza joints deliver. I know a guy who has carried 18 large pies without a rack. Six hanging from each grip and six on top of the handlebars. Best left to the professionals. Don't forget to tip!

I carried 18 pizzas once (from here, http://www.eatyourpizza.com/), but I have a trailer. I did not receive a tip. That equals 0% of the $381 dollars they spent on the pies. Never again. You want Pizza Paradiso, go get it yourself. Don't call me. End of rant.