The few possibilities are Tyvek, Nylon, or polyethylene plastic.
Tyvek is fairly waterproof, fairly cheap, and wears OK. Nylon has lots of varieties, but if you have a fully-coated nylon like Sil-nylon, that works. It costs more, but it wears better, and probably weighs slightly more. Plastic comes in all sorts of thicknesses. It is cheap, extremely waterproof, but is easily punctured. Depending on the thickness, it may or may not be the lightest.
The nylon fabrics are probably best if you are going to turn up the edges and corners and sew it. Tyvek can be sewn, but plastic will tear.
For one heavy-duty purpose, I made my bathtub floor out of coated nylon. It's heavy. For a quick/disposable purpose, I made one out of 0.7 mil plastic, but I expect it to last no more than a week or so. Previously, I had made one out of 1 mil plastic, and it lasted much better.