The best fabric fake I use is brown or white coffee filters. They're strong so they don't break when soaked in water. Cut narrow strips and dip in white craft glue, then layer them on the figure. You can slide them around and smooth/add wrinkles as needed. And you can "bend" it around corners and curves. You can even let it harden slightly, then lift it away from the figure to dry. That gives a 3D effect like when your t-shirt doesn't like flat on your neck, but sticks out half an inch. If it shreds, just peel it off and start over. It makes a great cloth-y knap that holds paint well and looks right.
For real fabric, the thinnest and smoothest I've found is white handkerchief material. It's practically see-through. Again, dip in white glue and layer.