I made a Westfalia style pop-top for my Land Cruiser, and for the fabric I ended up using Goretex. After much deliberation between canvas, cordura, and coated nylon it seemed that Goretex was the best option . While cotton canvas is cheap and quiet in the wind, it is not particularly water proof and is a real pain to deal with when wet. The coated fabrics are obviously totally water proof, but if it is wet (and cold) you end up with tremendous amounts of condensation on the inside of the fabric. The Goretex works great because it is totally water proof AND breathable and easy to work with. I also added two large zippered/bug screened windows to the triangular part of the pop top for a view and for when it is super hot out and I want cross ventilation. So far I have used it in snow and pouring rain with no moisture issues from inside or outside. The two possible draw backs to Goretex is that it is not as heavy duty as canvas type fabrics, and it can be louder in the wind. So far (~50 nights) durability has not been an issue, and if it is, making a replacement tent is not such a big deal now that I have a pattern to work off. The fabric flapping also hasn't been too bad as I cut the tent with a catenary curves on the seams and the roof is pushed up by gas shocks which assures the tent is super tight all around.
You can buy various types of Goretex in bulk from a couple of good outdoor fabric companies such as rockywoods or seattlefabrics.