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.

ETA - ease of closing again is just as important as ease of opening.