Not all insulators will work
With low pressure, insulation relying on trapped air is nearly useless. This rules out fibreglass, wool, polyurethane foam, and even aerogel. Since radiation is the only source of heat loss at altitude, you need radiant insulation. Layers of infrared reflective material separated by a small gap performs quite well on satellites. A cheap version can be made out of a space blanket and some cheap lace.
Of course, you'll probably need a bit of conventional insulation before it reaches altitude.
Biting the hand that feeds IT
