The secret of making nice obentos in minimal time is to prepare as much as possible in advance so you're not rushed in the morning. Cook as much as you can ahead of time, then either freeze it in small, one-serving bags, or put enough for a few days' obentos in the fridge (or freezer) in an airtight container. In the summer, it's not necessary to heat the food up before putting it in the box, but in the winter, it's a good idea to heat it up a little first. It's going to be about room temperature when eaten in either case.
It's also helpful to keep on hand little things that you can add in if you have a little extra space, like frozen edamame, either in pods or not. Another handy thing to have on hand is Japanese mazekomi rice seasoning packets, which are used to mix in with the rice before either making rice balls or adding the rice to the obento box.
My obentos didn't always look like this. Poor older siblings had to deal with a plastic container filled with rice with hastily-made burgers on top. I shake my head in shame. At that point, my only reference point for teenage boy obentos was my brief stint teaching at a sports high school where the boys brought just that to school. Probably not the best example to go by.
Of course, anyone can see insanely intricate obentos just by doing an image search, but my goal is not so much beauty as variety and nutrition.
Click this link to see some of the many Japanese Obento boxes available from Amazon.