Many houses burn to the ground by just a few embers that fall onto the roof. My design would resemble an oversized covering that would resemble a shower cap. The material would be metalized such that the embers could land without starting the roof on fire. Heavy elastic along the edges of this covering would hold the material down like stretching a shower cap over your head.
Simple and possibly reusable. Fire prone areas may stock a few at their civil defense shelters or with local fire departments.