Completely different "Cajun microwave" thread

Call it what you want, but I'm looking to make my own. However, this one is way different from the usual thread. I have a truck toolbox that isn't being used and looking to turn it into a Cajun microwave. Currently taking off the lid. It is slightly curved around the edges when inverted which should be perfect for holding the charcoal. I need your best ideas to keep this thing insulated while I test to see if the inverted lid is a somewhat snug fit or not.