Hi
I use 70 micron Drafting film from a Drawing & Drafting supplier here in Australia to make my racing sails. It's ideal and holds a good shape. The only catch is that it creases very easily if you treat it roughly. Most of the racing boats here in Australia use it in varying grades. For beginners making sails, a good idea (I think) is to borrow a set or use one you may have and trace the panels with the drafting film, then cut them out leaving a bit to spare on the leach and at the bottom to allow for adjusting size if your joins aren't perfect. You then use double sided mylar tape and stick it on one side then curve the other pannel to fit and have the edge of the film level with the edge of the tape. Then lay the joined film over the original sail and mark where the edges should be and cut it with your hobby knife of scalpel. Sticky back dacron is ideal for the reinforcing at the corners. Once you've made some this way, you'll get the idea of how to draw up your own. (Just a suggestion-it worked for me)
Jason