I use polycro, otherwise known as Window Insulation Film. Light, packs small, cheap, re-usable. Mostly to have something on the floor in the vestibule (reduces condensation, plus nice to have a dry floor). If you ever got wet knees from kneeling in your inner and finding that the water column of the ultra-light fabric is not up to the pressure, this helps too.
Careful with heat, it shrinks...