I sandwiched several layers of 1/8" foam to fill in the areas around my windows glued together with spray glue. I glued black fabric over the pieces. It does a great job of insulating, much better than a layer of reflectix. The layers have not stayed completely glued together so they squeak when I drive and from the outside the black fabric just looked like black fabric, not limo tint like I was hoping.
If you have tinted windows it might look better from the outside. If your windows are relatively flat you could probably just trim a block of 2" foam to fit in the opening.