Maybe make a wider platform for your rear rack out of a cookie sheet or something similar. Cut holes in it to strap it to the rack, then a couple of straps going around each way to secure the pizza bag. I actually did something somewhat similar using the bottom piece of a wire basket/box. It was about 2'x2' flat grid of wires(like a piece of shopping cart), so connecting everything up with the straps was super easy. I haven't tried the set up for pizza, but, now that you've got that idea in my head, it is inevitable.