Just Bento

With that particular box if you use the bottom layer + lid it will not be anywhere near sealed - the bottom of the top layer is the lid for the bottom part that sort of seals it, although not waterproof-ly. If you use the top layer it will be sealed but not pretty, because the pretty lid will be very loose if you put it on, so you won't. Some other boxes are better for using only a single layer, like http://www.bentoshop.de/Bento-Boxen-fuer-Erwachsene/Bento-Box-Urara-oval... (you can use just the top layer, although you'd need a tighter band) or http://www.bentoshop.de/Bento-Boxen-fuer-Erwachsene/Bento-Box-Hanamonyou... (I think you could use either layer, although also with a tighter belt).

Thank you, Bronwyn (love your name, by the way)! Then I guess I won't use that box for breakfast, but it's fine; I found a Tupperware one in our pantry that's just the right size. Concerning the other ones: I thought about getting the first one you'd linked, but it can't be put in the microwave or dishwasher, so I decided not to get that one. The other is a little too expensive for me. But thanks for linking those!
Oh, and you can actually use just one layer (or have one tier nest inside of the other one after finishing your lunch) with the other two boxes and the same belt, you just have to wrap it around differently (turned by 90 degrees... if you know what I mean).