So my kids like to take 'sushi' (that is, variations without raw fish) and sushi-like dishes (onigiri/onigiraza, musubi etc) to school for lunch. However in the mornings I don't have time to fully make them. So I have to make them the evening before. Over the, say, 15 hours between preparation and consumption, the rice tends to dry out quite noticeably. Are there any tricks to keep the rice fresh longer? Is there any difference between using plain vs seasoned (rice vinegar, mirin) rice? I've tried wrapping the dish tightly in cling film, which helps a little, but not as much as I would have expected. I've also tried doing prep (including cooking rice) the night before, and assembly in the morning. In that case the rice is less dry (I presume because the surface area is smaller) but by the time it's lunchtime, most gains seem to have been lost - plus it's quite time consuming/stressful doing even only the assembly in the morning.
I've thought about making the rice more moist but I'm not quite sure how to go about that in a way that it doesn't turn it into a mush, and most of the excess water would be absorbed by the nori anyway - which brings its own set of problems. (the nori going soft is already a problem when storing for more than say an hour)
Are there any other tricks?