So true about trying to stuff them back in once they have been used. The AMK Heatsheets Emergency Bivy, small and compact, weighing about 4 oz, can indeed be repacked, although it does take care and a certain amount of time to carefully fold it while compressing all the air out. It is part of my standard kit, along with something for a tarp and some light twine.

Another consideration when doing an emergency bivy is picking your spot with care. Moving just a few feet can make an enormous difference in exposure to wind, cooler temperatures, etc.