I have that food warmer and when I make extras for lunch I put them in plastic containers, such as this:
Then I just empty it into the aluminum container. I figure they are less expensive than the aluminum container, so I don't care if I accidentally lose one during transport, plus they stack together well so I use less cabinet space.
Sometimes I will even freeze my extras and take those. The frozen foods are the perfect size to fit in the container and if I plug it in as soon as I get to work it is usually thawed and heated through by lunchtime.