If you aren't successful making the home made version, commercial versions called "Frost Shields" are available for truckers in Canada, Alaska, etc. I still have a few in a box from Wave flying years ago, the box shows their address: "C.T. Enterprises Ltd, 1635 13th St, Brandon, Manitoba R7A 4S7 These have a sharp ~1mm step in the plastic 1/2" from the edge. The label lists several sizes, mine are 7" x 13". They worked very well at 23,500'.
~Barny