For the type of vessel you are making sails for you need egyptian cotton, available from this gentleman
http://www.modellingtimbers.co.ukDo not cut the main where it fits the mast in a straight line but a slight curve, this will give the sail a bit of shape. I make sails for one metres, southwinds, northwinds and all makes of model but do not do anything like you want. I have used draughting film for a gaff cutter I built these with properly shaped seams and it sailed well, though not quite looking the part. But the egyptian cotton is the way to go, about £3.50 per sq metre.
Cut the cloth so the selvedge runs up and down the leach of the sail, this will help to prevent the sail stretching out of shape when powered up., This goes for foresails as well.And you won't have to make a turnover seam here which will spoil the shape and the windflow. All other edges can be sewn over and some bolt rope sewn to the edge to hold shape.
Regards Andy Windpowersails)