Stop trying to make lunch boxes hold soup
For soup, the best leak-proof solution is a wide-mouth thermos, not a lunch box.
I’ve tested the “leak-proof bento” thing and it never works for actual liquids. Those silicone-sealed compartments are fine for yogurt or apple sauce, but real soup will find a way out the moment you tilt it in a tote bag. The only thing that genuinely survives being tossed sideways is a vacuum-insulated food jar with a screw-on lid and a silicone gasket.
The classics are still the best: Thermos Stainless King, Hydro Flask Food Jar, or Zojirushi. All three have wide mouths so you can get a spoon in there (important), and they seal completely. Bonus: your soup stays hot until noon, which is more than any plastic bento can offer.
If you want to carry a sandwich plus soup, use a regular lunch box for the dry stuff and put the soup in its own small thermos. That’s the only setup that didn’t leak on me.
Your backpack will thank you.
