On my Xtracycle, I just put the pizza box on the Snapdeck and wrap two of the straps over it. It's worked fine on somewhat bumpy 2-3 mile rides. I'm sure if I were a better planner I'd come up with some elaborate devices for all the various stuff I haul, but really the "plain vanilla" Xtracycle does a pretty good job for just about anything.

The other option, of course, is to carry someone on the back and make them hold the pizza!