I was looking to do the same thing, but with a large family-size lasagna. I'd probably do the same thing Tourister suggested and use some plywood strapped to the rear tray. I ended up buying some frozen lasagna from the supermarket instead because I didn't feel like going to all that trouble. If I ever do pick up a large pie or a big tray of food, I'd do it at night when there's less traffic and less people to see me.