I have stocks of Dacron, cannot remember the weight, I think about 120-150 g/m2.
This is commonly used on working models (get the bottom wet) and it has a flat even surface with not pattern. It is available in colors, including cream. I have pencilled in lines to mimic seams in the sail cloth.
It is stronger than nylon, it will not rip, and you can use metal eyelets in the corners for attachment points.