Plastic containers inside a plastic/Ziplock bag so that if it does spill the mess is contained, more or less. Glass is too heavy as others have said. So is a thermos. Gotta be willing to compromise.

I'm a creature of habit, so it's oatmeal every day. I make it at work with mostly everything already there.

You'll figure some kind of system as you become more accustomed to the commute. It's easy for us to tell you what you need to do, you have to figure out what works for you.