I would try a putting few drops of dishwashing soap on a paper towel, then put that in a Ziploc bag with your lunchbox.
One more idea that I got from cleaning cast iron skillets in my home. You're not supposed to use soap on a cast iron skillet because it can remove the seasoning. The recommended way to clean them is to use salt and water. From what I've seen a paste of salt with a few drops of water will do a pretty good job of removing excess oil from a cast iron skillet, it doesn't take off 100% though. It would probably also work for your lunchbox, it's kind of abrasive though and would encourage rust depending on the material the lunchbox is made of.