how to store rice for 12 hours so it doesn't dry out

For eating rice cold, I find it best to cool it by rinsing with cold water as soon as it's cooked. Then drain well, pack into a close fitting container, and refrigerate. It also works well if you plan to reheat the rice.

You may find that you want it a little softer than at dinner the previous night. In that case let it sit hot and wet while you eat dinner, then drain before the cold rinse.

This rapid cooling, apart from preventing the rice from steaming in the box, is also good from the point of view of food safety.

I have a sister-in-law who has gluten intolerance, so she travels with plastic storage bags with rice in them, in case she arrives at a destination or airport where non-wheat convenience foods are not easily available. I'm not sure if she further wraps smaller portions in plastic wrap. This way, when it's in the bag, she can squeeze out every last bit of air out of the bag before/while sealing it. This minimizes air flow and keeps it more moist, I think.

Definitely do not put it in the fridge if you do this, because once it gets cold, the starches harden and need to be heated. Room temperature and this bagging seems to work for her, though I haven't asked her about it in great detail.

She also uses a stickier medium-grain rice instead of a long grain, just because it seems to retain a bit more moisture.

At our home we regularly eat leftover cold rice the next day with breakfast or lunch. The trick is to be sure that it is not exposed to air and not refrigerated. We leave it in the pot that it was prepared in, being sure to keep it covered/sealed. It stays perfectly good through the next day - in fact we usually don't take it out and put it into a container for the fridge until the next night's meal. Additionally, I also shake off the condensation which gathers on the lid after cooking. I'm not sure if it does anything, but I feel that excess moisture may lend to faster spoilage.

It should be noted that this is using Japanese/Korean rice which usually a medium grain Calrose rice which characteristically softer and stickier than Chinese or other rices. I can't say what the results are like for other drier rices.

Also, if you use the type of cooker that has a warmer control on it, Japanese/Korean households leave it on constantly so that warm, if not hot rice is available at any time, so that they are able to pack rice while it is still warm in the morning for the day's lunches. Maybe you might want to invest in one of those...

It has been my experience that adding water before microwaving helps it to spoil faster if it is not used up completely (eaten). The best method for reheating leftover rice is to heat it sufficiently - approximately 1:30 mins per cup - covered and without adding water. (This is also the time instruction for pre-cooked rice available in individual servings packages in stores.) It comes out like freshly cooked. Well, nearly.

Putting leftover rice in the fridge immediately makes it hard, which is easily remedied with a microwave, but when it's not available as the OP stated, the above method works for me.

My son's trick is to put however much (leftover) rice you want into the microwave with a small bowl or cup of water. You only need a couple tablespoons of water - 1/4 cup at most. Microwave the rice long enough to heat it up; the water will 'resteam' the rice (making it moist again).

As an experiment, you might try leaving a gap between the rice and the sealed lid, and putting a piece of clean, absorbent, lint-free fabric, ( muslin would be good ) or paper towel, in the gap. If the container is appropriate, you can stretch the fabric over the underside of the lid, trapping it in the seal. This will help keep the humidity even, prevent a condensate forming and dripping back into the rice.

I would try this with dry fabric to begin with. If you're still losing too much moisture, you can moisten it first.

This is an extension of one method of preparing (long-grain) rice, where it is allowed to finish cooking in its own heat in a very warm oven, similarly sealed.

(Salad leaves keep for longer in the fridge, sealed with a dry paper towel.)

When I cook my rice, after it is cooked, I spread it out on a sheet pan and let it cool completely before placing into plastic sealed containers for storage. I've never had a problem with this method. If the rice isn't cool, you will get that condensation inside the container.