I build both scale rc racing yachts and some scale gaff rigged cutters...all actually sail RC. I use several different materials, for the cutters, I have found the light spinnaker material from sail shops works great, much lighter than you would expect.
I live on the US west coast near many sailing harbors, the sails shops just give me some of their scraps, free!
Light weight reinforce, checked pattern, from the local fabrics stores work also. Like a very light windbreaker material.
Hope this helps.