I usually use panniers, and they work great for me on a daily basis, but I understand what you mean with potluck dishes. Once I took my food to a friend's house by leaving the panniers at home and attaching a plastic file box to my rear rack with zip ties, and then putting the food in the file box. It looked a bit stupid on the way, but it worked great, and much better than the alternatives: taking the bus, bumming a ride from someone else with a car, or putting the food in ziplock containers and using panniers. (Being a guest at a potluck or other social event is a lot more enjoyable if you're not the eccentric car-free guy who's also a pain in the ass... )