The rice is Japanese/sushi style rice, steamed, and the egg layer is just scrambled eggs with a drop of soy sauce, made in a pan (add a little oil to start with) and cooked until they becomes quite dry. I lined a box with grease proof paper, then place half of the scrambled eggs in the bottom. Then half of the rice, then the rest of the scrambled eggs, and then covered it with a final layer of rice.
Press down well and let it cool down completely.
Tip the box over and there you are, some square egg onigiri ready to be cut. I have to say that he preferred this one! Me? I don't know... I think I still like the ume best, but I must say, this was actually very nice and effective, and I will be making it again!