We also use a dual compartment lunch box from Laptop Lunches:
I like that it keeps the bottom portion flat so the food is not shifting all over the place. There are probably other lunch bags that are the same size, but this is what I purchased. I'm really happy with the quality/price and their customer service is top notch.
I am currently using a Ziplock Divided Rectangle container (thanks to Lisa's tip at 100 Days of Real Food) in the bottom portion of the lunch box. I am loving how it fits perfectly in the bag and is only one unit to clean every day. We have two of the Laptop Lunches container units which are very nice (great quality), but come with a lot of pieces to clean and/or lose every day. I think they will work as my daughter gets older and starts to care more about her lunch looking "cooler". I don't know. But for now, the Ziplock containers work and are much cheaper and easier to deal with.
I'm always open for suggestions/tips/ideas - let me know! :)