Insulated window coverings for cold weather

What you are looking for is not fabric but Mylar - Reflectix. It is sold on a roll at Lowes in the insulation and heating section. It is Mylar with bubblewrap sandwiched in between and is used for insulation. You cut this yourself with a pair of regular scissors a little larger than the dimensions and shape of your window. It can be pushed into the inside window frame and friction fit or you can rig up Velcro to attach it around the window sealing it off. We have this in our RV. It keeps the cold out in the winter at the windows and in the summer keeps the heat out the same way. Not sure why no one else has suggested this. Many RVs use it just this way. We have some windows that we just leave it in, and some that we put it in and out of in just a few seconds each window.