USE AT YOUR OWN RISK! RESULTS WILL VARY! I'M NOT RESPONSIBLE FOR HOT CHEESE IN YOUR LAP! Transporting a pizza is difficult when you're driving alone. A fresh pie is fragile, if kept at an angle the precious cheese and toppings will congregate at one side leaving half the pizza naked. In extreme cases a gooey pool can form inside the box and at that point the patient is dead. One day I got in my Prius and set a pizza on that mini desk of a dashboard and then noticed that the corner of the box fit neatly into the slot there. I've never looked back. Small, medium, large, think crust, thin crust, Chicago, they all fit because just about every pizza box is the same thickness. The attached picture illustrates the principle and despite the skew in the photo the box there is pretty level. It takes a quick moment to position the box just right and I find that some sort of angle gives me the best chance of getting it sitting pretty. With a little practice you too might be able to make use of this unknown, secret, clever design element.
That looks like a disaster waiting to happen. What's wrong with the floor of the passenger area ( if available) ? I've even placed a pizza in the rear hatch area. Level, flat, safe. I think there are many other options. Wedging it in the dashboard? Think I'm going to pass on this hint.
The floor of the passenger area if no passenger. The Prius Horizontal Net in the hatch area. Dine in.