I think denier is thickness. I know that 1200 denier cordura is thicker and tougher than 600 denier.
What I used when I made the sheets was a fabric called Supplex and it is water-resistant and wind-resistant. It may have been wind-proof but it's been a while and I don't remember. It is available from The Rain Shed in Corvallis, Oregon. Probably available elsewhere also, but I've purchased from them. They have all kinds of fabrics used for outdoor sports and equipment. Nice shop and great people. Good prices, too.
The trail is the thing....Louis L'Amour