I use paper cups for cupcakes as dividers. I just throw them away after it.
There also is a collection of blue Urara boxes.
...a triangle shaped box for onigiri lovers
But it can fit triangle sandwiches if you get tired of large portions of onigiri ;) Here's an example.
A square shaped box.
Photo from here.
All three shapes comes in red, blue and yellow color. And I'm thinking about ordering the onigiri and square boxes in the future ;)