Experiment: anchovy onigiri was interesting, but perhaps not entirely successful. I have a couple of tins of anchovies I need to use up, and on the grounds that salty fishy fillings go well in onigiri I set about making some. It's certainly salty and fishy, but anchovy doesn't seem to meld with rice the way smoked salmon does. Though I'm liking this second one better, so it may be a matter of absorbing the new taste. The ones I just made with the bonito topping on the outside were a little bland. Hmmm, checking the bottle, it doesn't have much salt in it -- possibly I need to add a little sea salt to make things pop.
However, I can strongly recommend this method for making onigiri. I got tidy little onigiri, already wrapped, without getting rice all over my fingers.