We just bought an insulated lunch bag which was a good shape for her bag. We will continue using a variety of smaller containers inside, depending on what we're packing for that day. In the past I've found it too much of a PITA to have a single lunch box because there will always be something that doesn't quite fit, whether it's an oversized apple or whatever. It also means that during winter I can pack some hot food inside a thermos and it will still go in the same lunch bag.
We do the same. Means lots of little containers, but it then DD1 is able to just take out what she wants at each meal time. I don't want her dragging out her whole lunch to just get to brain food. Would rather the main lunch stays in the cold bag until needed.