I use a BringYourOwnBag.Com Insulated bag to carry my hot and cold food when riding my motorcycle. I stuff the bag inside my backpacks largest compartment.
The Bag consists of a Nylon Exterior, thin Foam Insulation, and a Reflective Mylar Temperature Barrier. It is a leak proof bag that zips shut across the top of the bag.
I also carry a few spare plastic grocery bags so if something looks like it may leak, I can tie another bag around it to keep the various pieces of my lunch or dinner from getting everything else soggy! :-)
You can find these type of insulated reusable shopping bags at just about every grocery store chain in North America under the store's own name.
The mfrs website is http://www.instorecorp.com/byob_about.html so you can read up on the different types of insulated bags they manufacture.
