We have found the head of our bed a bit cooler than the bedroom during cold weather. The headboard wall is basically the other side of the outside rear fiberglass of our coach.
During our replacement of interior fabrics, we changed headboard fabric. The headboard slips up from two wall brackets after removing three screws located under covered snaps.
While headboard was off, we painted the unfinished wall under the mirror and then fitted a ½" thick piece of Styrofoam to insulate the wall. After the re-covered headboard was reinstalled, another piece of Styrofoam was fitted over the first Styrofoam in the space under the headboard. We should be better off in cooler weather with 1" of insulation in the open space in front of the mattress. All the insulation came from a 4' x 8' inexpensive piece of ½" foam board from Lowe's.
Barry and Cindy, the heat in our bedroom has also diminished considerable in the last few years. Your idea is the best that I've seen and will try it. He! he! he!
Maybe a few more mirrors will help! Nice job on the insulation I intend to shamelessly copy your effort.
Chuck