The thermos is your bento's best friend.

Packing a bento with both hot and cold food works best when you stop trying to do it in one compartment. You don’t need a fancy temperature-controlled box — you need a thermos and a cold pack.

The hot food goes in a vacuum-insulated jar (a small Thermos works great), and the cold food goes in the main bento with a small ice pack. The thermos does the work of keeping heat in, so nothing else in the box gets cooked. Just don’t let the hot jar sit directly against the cold stuff — give it a little gap or use dividers, since the outside of a thermos stays warm and will work against your cold pack.

Two tricks make this much more effective. First, preheat the thermos: fill it with boiling water for a minute, dump it, then pack your hot rice, curry, or soup. Second, fill it to the top — air is the enemy of hot food. For the cold side, frozen grapes or cherry tomatoes work great as edible ice packs; they keep things cool and thaw by lunch.

Timing matters too. Pack the hot food right before you leave in the morning, and don’t open the thermos until you eat. A good vacuum jar keeps food hot for 4-6 hours easily.

Your lunch doesn’t need a $60 gadget. It needs a decent thermos and some frozen grapes.