On my twin cam special I just made a replacement in aluminum, no insulation and the trunk doesn't get hardly warm. I do have a rug too, but really, not much insulation needed if your muffler is in good order. Saves weight too, steel isn't needed. Of course the al sheet is spaced about 1/4 inch from the trunk bottom for air circulation, and no thermal conduction to the fiberglass.
Jerry Rude
4005R