Just in case you weren’t aware, Jacks R Better’s Weather Shield is made of just that sort of fabic, for just that sort of purpose.
http://jacksrbetter.com/index_files/Weather%20Shield-v2.htm
If you do find the material, I think you will be happy with it. I found no condensation/frost inside my JRB Weather Shield bottom, while I did have condensation inside the silnylon HH UnderCover.