For a backpack side pocket, the best 'lunch box' is a food jar.

If you want a lunch container that actually fits in a backpack side pocket, skip the lunch box and get a wide-mouth insulated food jar.

Standard side pockets are built like water bottle holsters. A traditional boxy lunch box is too wide and flat to slide in, and if it does, it’s a tight squeeze that wrecks the pocket and the zipper. A food jar is tall and cylindrical, slips right in like a