We use the full stainless lunch bot uno, one for each kid.
LunchBots Uno Stainless Steel Food Container
They work great! I have no idea why it's so expensive right now, though?? I can fit the lunch bot, a Kids Konserve small and/or medium container and a kleen kanteen in their lunch boxes (1 LLBean Critter and 1 PB classic, the PB classic is a bit more of a squeeze).