Here’s my pizza setup, which I think would work. I cut a coroplast yard sign to large pizza box size (those are medium pies in the bags in the photo) and cut four slits so that I can attach it to my rack with some nylon straps and buckles I had lying around. Pizza boxes go on top, usually inside foil insulation bags as you see here, and are held in place by a cheap motorcycle cargo net like what LhasaCM suggested. I moved two of the six hooks on the net so that it hooks on each of the four sides.