I went to Japan this year and became addicted to onigiri for breakfast. All the convenience stores carried them in the morning and they were nice and filling. I bought a mold for them when I got home but shaping them by hand isn’t bad either. You can fill it with whatever but canned tuna or salmon with kewpie mayo and soy sauce is really easy. Plus, you can eat them cold so less prep. Not sure how they would work with meal prep though (especially if frozen then defrosted) but might be worth a shot since they are cheap to make.