Had a Japanese Onigiri dinner with the fam and used a Tupperware rice ball maker to make Onigiri with all sorts of filling. I had 3 of these *burp* .
You can make your own Onigiri/ Mini Sushi Cake in any kind of container by coating it lightly with oil and then lining it with cooked sushi rice, making a well in the middle and filling it with any filling you like and cover it with more rice. Use a spoon dipped in water to spread and press the rice. The water on the spoon prevents the rice from sticking to it. Unmould on a serving plate. Top the Onigiri with strips of Nori and Tobikko (Flying Fish Roe) or Ikura (Salmon Roe).