Styrofoam is an excellent idea. Your experimentation is super valuable.

Another method I’ve found to really work well with very light weight and low bulk is Reflectix. A pot cozy made with Reflectix keeps a hot pot steaming after a full hour. It is fairly easy to construct a Reflectix sleeve that will really perform. That is an inexpensive alternative. A double layer of Reflectix is hard to beat.