Cut the fish into thin slices (1/8-inch) against the grain. Using the outside perimeter of the oshibako as a template, place it on top of the sliced fish and cut the edges of the fish to fit inside the box.
First put a sheet of nori inside the oshibako, and top with the first layer of fish. Then evenly spread a layer of 1-ounce of snow crab. Follow with a layer of 1 teaspoon tobiko, then 1 teaspoon mayonnaise, and a layer of 1 cup sushi rice. Finish with another layer of fish. Then press the oshibako lid to compress and mold the sushi.
Invert to remove and cut into 6 even pieces crosswise and in half lengthwise to yield 12 pieces. Repeat process for 3 more times to yield 48 rectangular sushi. Keep sushi cold until ready to serve.
Note: This recipe can be used in conjunction with the other recipes for the elements of the Bento Box of sushi hors d'oeuvres made on the episode which also included: Rice Balls, Maki, and Nigiri Sushi. Also refer to the Sushi Rice recipe as needed.