Take some rice vinegar and wet your fingers and the wooden mold with it to prevent the rice from sticking.
Assemble the box so the bottom base and sides are upright and the top of the box is fully open. (It looks like a box with no lid).
Slice your salmon, avocado, and cucumber into thin strips.
Start to add the smoked salmon to the bottom of the box.
Then, add a layer of sliced avocado.
Next, add a layer of the sliced cucumber.
After the toppings are layered, you need to cover them and fill the box with the sushi rice just below the rim.
Spread the rice out evenly and fill the corners.
Using the top piece of the oshibako (the third piece), press down the rice firmly.
If some areas are sparse or uneven, spread the rice out to make sure it's spread out evenly. Continue to press down firmly.
After pressing, slide the side part of the box up. You should still apply pressure to the top of the press.
The result is layered sushi. Flip over the sushi box on a large plate and then slice the sushi into small rectangular bite-sized pieces.
Drizzle the kewpie mayo on each piece.
Serve with soy sauce, pickled ginger, and wasabi paste.