Get a bottle-shaped lunch bag, not a box.

Yes, there are lunch bags designed to fit a backpack side pocket – they’re basically insulated cylinders that slide in like a water bottle.

Standard backpack side pockets are made for water bottles, not Tupperware. So the lunch carriers that actually fit are soft-sided, tall, and tapered. They have a zipper top and a wide mouth so you can pack a sandwich, apple, yogurt, whatever, and they keep it cold while sitting in that pocket. The main compartment stays free for books.

“Perfectly” is the catch. Side pockets vary a lot: some are stretchy mesh, some are rigid, some are too shallow. Before you buy, measure the pocket’s opening and depth, or look for a bag that lists its dimensions. Most bottle-style lunch bags are around 9–11 inches tall and 4–6 inches wide, which works for the average pocket.

If your pocket is too small, don’t force it. Clip the lunch bag to your backpack’s compression strap or daisy chain and call it a day.

Your tuna sandwich deserves a secure ride.