Cut the milk carton about 3 cm from the bottom.
Cover with plastic wrap.
Mound in the rice. (Optionally add salt to the rice beforehand. If you are adding a lot of filling you don't need the salt.)
Make a dent in the middle.
Add the filling.
Cover with plastic wrap.
Pack down gently. If you want firm onigiri, press down firmly.
Done!
Optionally wrap with nori seaweed (The filling here is umeboshi).
If you want to wrap the whole onigiri with nori seaweed, use 1/4 a sheet and place on top of the onigiri.
Wrap.
(The filling here is flaked salted salmon.) Since you can see the filling, I think just wrapping the onigiri on one side with nori is fine...
(The filling here is konbu tsukudani.)
See for a rectangular onigiri: https://cookpad.com/en/recipes/144899-easy-rectangular-onigiri
You can make small squares too.
