There is another application for signal mirrors. They make very good light sources if you want to investigate dark cave entrances or similar. they are excellent at throw and illuminate the area in which the beam strikes quite brilliantly. I learned this trick when investigating mine shafts for a missing Park Ranger some years ago in Arizona. The Mine Safety guys came out with a recycled medicine cabinet mirror which easily lit up lower levels of mine shafts 100 feet or more distant.

Of course packing around a cabinet door doesn't sit well with the lightweight philosophy of this forum, but smaller sizes will also work well.